Long lay over in Seoul. Do I need a visa as a Filipino citizen?

I am sorry, but it appears that the answer above might not be correct anymore.

According to Consulate General of the Republic of Korea in Boston, it seems that you should be able to visit Korea without visa:

Although citizens of the countries are not under Visa Exemption Agreement, visa-free entry for a limited time is allowed under certain conditions. They include:

• Citizens whose final destination is the United States, Japan, Canada, Australia or New Zealand and have a valid visa (re-entry certificate) for those countries.

• Citizens who departed from the United States, Japan, Canada, Australia or New Zealand and transiting in Korea in order to go to their home country or a third country.

Entry Requirements

• Must have a departure ticket from Korea within 30 days from entry
• Citizens must not have any record of illegal stay or violations in the United States, Japan, Canada, Australia or New Zealand

So if you fly FROM USA to Philippines (or the other way), AND have a valid US visa or residence permit, AND arrive on a direct flight from USA (or departing on a direct flight to USA) you should be able to enter Korea for 30 days maximum.

This is further confirmed by Timatic (note that I put in Korea as a destination country, not a transit country):

Visa required.

The following are exempt from holding a visa:

Passengers with a residence permit issued by USA to nationals of Philippines if in transit through Korea (Rep.):

• holding confirmed onward tickets on flights departing within 30 days; when
  • arriving on a direct flight from USA, departing to a third country (e.g. USA-ICN-PVG); or
  • arriving from a third country, departing to USA (e.g. PVG-ICN-USA). Not applicable if traveling from/to Northern Mariana Isl.

Thus the main requirement here seem to be to have a direct flight from/to USA (i.e. you cannot get visa-free entry if you fly USA-Japan-Korea-Philippines according to Timatic)