Yeah, unfortunately R.E.M. are indeed in the studio ( and I don't mean unfortunately as in "Oh no, they're making MORE music?!" because I love R.E.M.

) so they're pretty unlikely.
When I read KISS there, I had a sudden "Bingo!" moment, but it's true that their dates make it pretty hard for them to be there.
U2 are nigh on impossible, unless Bono can break time barriers. Shame too, would be my #1 choice for headliner out of some of the options available.
Muse are rumoured to play Glastonbury, as are Coldplay, but I think both are unlikely due to the fact that Muse are already playing 2 huge dates in London this year, and Coldplay played last year so I think that would weaken ticket sales considerably and they wont want that.
G N' R... I'm not sure... I used to like them, but I've heard terrible, terrible things lately... It wouldn't be the worst choice, that's for sure.
Sure hope it isn't Fleetwood Mac anyway, I've seen them on so many "Worst gigs of '09" lists, that would be a total bummer.