NBC has yet to officially comment, but it makes sense for Carrey to show up in the May 19th episode, as he has Mr Popper's Penguins to promote.  That film opens nationwide on June 17th, along with the superhero event movie Green Lantern.

Jim Carrey joins previously announced finale guest stars Rickey Gervais, Will Arnett, James Spader, Catharine Tate, and Ray Romano, all of whom will be vying for the managerial position vacated by Steve Carell's Michael Scott.  Gervais played manager David brent on the original British iteration of The Office, which he also co-wrote and co-directed with Stephen Merchant.  Gervais has publicly stated that he believes Arnett should replace Carell, but it's unknown whether any of the guest stars will actually end up with a permanent gig on the popular sitcom.  You can count out Carrey and Gervais, though.