Blue Sky Disney has released more information about a potential Monsters, Inc. sequel. Back in July there were some more rumors concerning a possible sequel, but, according to Blue Sky Disney, the sequel is all but confirmed by Pete Docter (the probable director due to his directing of the first Monsters). The supposed release date will be some time in 2013.
I don't think that this rumor is too far fetched. Afterall, Pixar is doing a lot of sequels already Cars 2, Toy Story 3...). I think that a sequel to Monsters, Inc. has a lot of potential, especially when executed by the geniuses at Pixar. I guess we won't know for about another year or so when the definitive announcement is made, if it is made at all, by Pixar.