Entertainment Tonight and Access Hollywood both said tonight that Brad Pitt will guest star on Friends (so he IS the mystery guest in 809)...they say his episode will air in late November Sweeps (Thanksgiving). They said he plays Ross and Monica's geeky high school friend.
Also, Pat O'Brien was on the set for the episode and they showed clips of him walking outside the Central Perk set with David Schwimmer and the preview for tommorrow said: "We are on the set for Brad Pitt's episode of "Friends", plus we talk with David Schwimmer about what it will take to keep the cast together. I don't know about you guys, but that sounds pretty optimistic to me. I will definitely watch tommorrow's Access Hollywood, but it seems the way they phrased that, that the cast may be willing to continue the show past this season. If David says in the interview, "no, this is our last year", then the preview would say something like "David Schwimmer talks about the end of FRIENDS." This way they said it makes it seem more like the cast isn't all dead-set against one option, that they are still pondering.