This was actually tough. I tried to include people who
might actually enjoy playing. As much as I'd like to include Denis Leary or Dennis Miller, for example, they'd spend so much time making fun of the situation, we'd never get any playing done.
Anyway, I thought this list would be lots of fun:
DM- Stephen King (running a Ravenloft/Cthulu hybrid w/ some Gunslinger flavor) OR Genndy Tartakovsky (running a Realms/Rokugan game) How's that for hitting the high points?
Fighter - Patrick Stewart (actually, I'd love to have him as a paladin. Which is really just an excuse to listen to Jean-Luc give moral speeches. Oh, and John DeLancie would pop in occasionally to play the campaign's god of trickery, ever the party's tormentor/benefactor/nemesis)
Wizard - Alyson Hannigan (just fits. Do I need reasons for all of them?

)
Cleric - Sam Jackson (he's got the *** whoopin' domain, but he's tryin'
real hard to be the shepherd)
Rogue - Bruce Campbell (which would be especially great with King/Tartakovsky DM tag team, though maybe he could get Sam Raimi to drop in and run a few sessions, too)
Ranger - Me (w/ bow feats instead of dual wielding, please)
Bard - Kevin Smith (the latter-day Shakespeare might as well be a bard. With some perform:mime and perform:slapstick for the lunchbox)