Four times makes it an institution ?
After the poster events at STOC 2011, STOC 2012 and SoCG 2012, FOCS 2012 makes it four times. Tim Roughgarden notes that the deadline for submitting posters to FOCS 2012 is in two weeks (Sep 11). So if you have interesting work you'd like to air out in the theory comunity, and a deep longing to visit New Brunswick, New Jersey, then submit your entry.
By this time, there's a good chance that you've already experienced the posters event either as presenter or audience at one of these venues. If not, I'll reiterate what I've said before: presenting a poster is a great way to disseminate your work with more personalized interaction than you often get with a paper presentation.