I was told my signature was ripped out of my passport. Should I be worried?

I'd ask if she had any blank cheques (checks) with her. A common trick is to remove a cheque or two from the middle of a book so it isn't immediately noticed. With a perfect sample of her signature, she could be set up to lose thousands of dollars from her chequing account.

This was no accident. On the positive side, the motives are more likely those of common criminals than of terrorists.

As her passport has been "modified" (or damaged) it is no longer valid, and thus needs to be replaced if she is traveling internationally again. Many countries will not grant you entry if a page has been removed as they believe it may be an attempt to hide previous stamps. Further, a US passport is not valid if it is not signed, so as the signature page has been removed it's certainly not valid.

As far as why the page was removed, this is not something I've ever heard of happening before. I suspect any attempts to guess what happened would be nothing more than speculation.

The passport has been SIGNIFICANTLY damaged. Normal wear and tear is generally acceptable, but missing pages or vital data or stamps missing - that's considered significant, and you'll need to get it replaced.

As a source, from the official FAQ page of the US Department of State and Travel:

If your passport has been significantly damaged, especially the book cover or the page displaying your personal data and photo, you will need to apply for a new passport. Conditions that may constitute damage requiring you to replace your passport include water damage, a significant tear, unofficial markings on the data page, missing visa pages (torn out), a hole punch, or other injuries.

Normal wear of a U.S. passport is expected and likely does not constitute "damage." For instance normal wear includes the bend of a passport after being carried in your back pocket or fanning of the visa pages after extensive opening and closing.

If you need to replace your damaged passport, you will need to submit the following in person (See Where to Apply):

The damaged passport; Form DS-11; and All documents required by Form DS-11, including citizenship documentation (i.e., birth certificate).

Should you be worried, or not? Was it intentional or not? Possibly. But without the photo page, there's not much anyone can do with it. Perhaps someone was trying to copy it, maybe someone slipped while thumbing through it - it's speculation really. So worried about the passport - yes, get it replaced. Worried about security / fraud - well there's not much you can do, aside from reporting it to the DST when applying for the replacement, as suggested on some of the embassy sites.

Looks like you have been victim of fraud/identity theft.

You should contact your government immediately. Although not all of the passport is gone, you might want to look at instructions for people who have had their whole passport stolen to get an idea of who to contact. If you've lost a document and don't know where it is, you cannot be sure that it has not been stolen and abused.

So, you have two problems right now:

• Part of the passport is potentially in the hands of a malicious person. They may attempt to use this to impersonate you with eg. banks or to commit crimes with your name for which you may be held suspect. You should inform eg. your bank as well, but starting the "stolen passport" procedure with your government will give you grounds for a case if you have to eg. deal with financial obligations caused by the malicious person.
• The passport is technically not valid. Customs officers of various nations, as well as some private companies who are required to check ID/signature, are supposed to no longer accept your passport as valid ID (although they may use discretion and accept it anyway in some cases).

As for the missing stamp, usually countries are more concerned with entry/exit through their own border, not other countries - but a government official, for instance a consulate worker reviewing a visa application (especially one from St. Lucia), may suspect that you have overstayed your visa in that country since there is no proof of exit (though you can easily explain that by showing the US entry stamp). You could probably get away with it unless you actually go to St. Lucia (which may be how your mother was even allowed to enter the US with an invalid passport), but it's best to have an official document explaining why you have an entry stamp but no exit stamp (because customs officers will want to know why). And if you do "get away with it" because one officer let it slide, another one could later find out and get you into trouble.

In the end, you definitely must notify the appropriate branch of your government that a part of the passport is missing and possibly stolen (and carefully document the procedure so that you have proof!). They will probably require you to obtain a replacement. It is also advisable to notify other important entities, such as banks (where identity theft may cause you great harm) that this has occurred.

This answer contains just one tip in addition to what's in the other answers:

I have heard of a particular kind of scam you should be wary of.

People steal your identity, contact your family pretending to be you or to be somebody looking after you. They tell your family a story about something terrible that happened to you and you need money to be helped, brought home, etc.

Since they have stolen part of your passport there's a chance of some kind of identify theft.

Please contact your family if you have not already, let them know there's a chance somebody might be stealing your identity. Tell them where you are, what your actual condition is, and what your upcoming plans are. Try to give them a way to contact you that should not be known by the person who had access to your passport.

Warn them to be suspicious of anybody they might hear from either pretending to be you, or bearing bad news about you.

From the fact the you do not identify the country of the passport (and perhaps the fact that you don't travel often :)), I will assume here that the passport in question is a U.S. passport. The other answers contains some useful and some useless information. I hope that the following is entirely useful AND may be urgent:

1. So far, only the U.S. government is the victim of a crime ( or perhaps several crimes). The passport does not belong to you, it belongs to the government. And according to you, someone has clearly stolen pages from government property which just happens to be illegal to deface. It's VERY unlikely ever to be a legal problem for you, but at the very least, won't you feel a tinge of guilt if something blows up tomorrow and the page that was stolen from you passport was somehow involved? Again, this is very unlikely, but the point is that you should report the incident immediately. (I will tell you how in a moment).
2. Even if SO FAR you are not the victim of a crime, as has been noted above, someone has surreptitiously and carefully stolen your signature. And not just your signature, but a copy of your signature that is known to be accurate and probably carefully done to be recognizable. You don't need a web site to tell you what to do, do you? NO need to panic or even rush, but think of things someone might do with that signature, and tell the people who might be on the receiving end of that fraud what has happened. ALso, not trying to be rude, but are there OTHER things that are missing that you perhaps haven't YET noticed? Such as those few inside checks, etc. mentioned above. If checks are missing, you might want to tell your bank, no? Do you have any credit/debit cards that you don't use very often, but keep with you? Ones that you haven't noticed until now are missing? If so, perhaps you should call the card company, etc.
3. Your passport is certainly invalid. They let you back in to the US because it was clear to them that it's ok to do that. But they will not let you out, because no country will let someone IN with such a damaged passport.

Putting these three things together: If she wants to be a good citizen and tell the US gov't about the crime -- if she wants to help protect yourself in case someone is about to steal your identity --

REPORT the incident immediately;

and if she ever wants to travel outside the US again

she needs to report the incident and apply for a new passport, using the procedure which is reserved for people whose most recent U.S. passport has been mutilated, altered, or damaged. I quote from http://travel.state.gov/content/passports/english/passports/renew.html

"If your most recent U.S. passport has been mutilated, altered, or damaged, you cannot apply by mail - you must Apply in Person."

See the following web page for how to do this: http://travel.state.gov/content/passports/english/passports/information/where-to-apply.html