As a service to you (as well as to help me figure out exactly whos confirmed, rumored, and not), here's the list of whats known so far.
CONFIRMED:
Jay-Z
Violent Femmes
Yellowcard
The Offspring
Yeah Yeah Yeahs
OAR
P.O.D.
Papa Roach
LostProphets
RUMORED (but confirmed off the record, according to AbsolutePunk.net):
Cypress Hill
New Found Glory
Modest Mouse
The Living End
Taking Back Sunday
Cypress, NFG, and TBS are pretty certain to me. Looking at the outlines of the pictures on WHFS.com, it seems clear to me now that the bottom middle picture is the outline of a well-known TBS promo pic. Ditto Cypress on the left-side second to bottom and NFG on the middle second-to-bottom.
If the rumored bands are all true, I think I'll strongly consider going to this (might make me the only user on this boards at the show). Jay-Z should be fun. I'm still a fan of Taking Back Sunday based on the days of basement shows on Long Island. Modest Mouse, NFG, and Violent Femmes interest me on a cursory level. Plus I'm a sucker for the festival atmosphere, which is why I go to Warped Tour every year to walk around the merch booths and sit in the dirt reading Milan Kundera novels.