This happened mere hours ago, with Pink Floyd's Gilmour (and drummer Nick Mason) turning up at Roger Waters' O2 Arena gig in London and guesting guitar (from the top of the wall) on Comfortably Numb. For one good thing he shreds the famous solo and for another good thing the quality (and sound quality) of this recording, done by a fan from their telephone at best guess, is surprisingly good too. Apparently this is only the second time or something that Waters and Gilmour have performed together since Waters' split from Pink Floyd and according to the write up in Rolling Stone magazine (that you can read right here) the crowd went suitably apeshit at the event.
Enjoy, it's a great tune.