Now there are many rumors out there, here are a few that I believe to be solid:
Tobacco (from Black Moth Super Rainbow fame): A Facebooker pointed this out on the Sasquatch! Facebook page. After a little digging found the info on Tobacco's Myspace page. Interestingly, it does not explicitly say that he is playing Sasquatch!, but Tobacco seems to be saying he will be there in his own way.
Grizzly Bear: Quartermass, the co-administrator, along with myself, knows someone trying to book Grizzly Bear, but there was conflict because they are playing Sasquatch! He also says he has on good authority that Monotonix will be playing Sasquatch! Thanks Quartermass!
Passion Pit: A Consequence of Sound reader and commenter sited that Passion Pit revealed that they would be playing Sasquatch! '09 as well as Lollapalooza and Coachela in an interview for Under the Radar Magazine. The issues cover is on the Under the Radar magazine website, but not the content, sure enough Passion Pit is featured as an artist to keep a lookout for in 2009.
John Vanderslice: A message board reader and commenter wrote that he heard John say on KEXP that he will be playing Sasquatch! this year.
Keep the rumors coming, if they seem solid I will post them!