Can I fly within the US if my ID is expired?

According to the TSA blog

If you’re 17 or younger, no problem… you don’t need ID to travel. If you’re 18 or older, no worries… you can still travel.

How so? Simply approach the travel document checker and let them know that you don’t have your ID.

You’ll be able to fly as long as [...] you’re willing to provide some additional information. We have other means of substantiating your identity, such as using publicly available databases.

And warns you that:

If we can’t confirm your identity with the information you provide or you’re not willing to provide us with the information to help us make a determination, you may not be able to fly. Regardless, if you do not have ID, please allow extra time for check in. We would not want you to miss you flight.

This flyertalk post reiterates the same: present expired ID, expect questions. And this post suggests even a Costco card was accepted (once. surely not official.)

YES, but...

1. An expired ID such as a Driver License is not a valid ID since it's is expired. The Screener many not notice this and accept it. You should not rely on this.
2. You can fly without ID if you allow the extra screening time. You should allow 2 hours for this.

So, your chances of flying are very, very good provided you have sufficient time for options 1, then 2 if necessary. Unfortunately, the only way to know is to try.

Details: TSA Identification