says that it may well be. The rumours (and we have to assume they are such until an official announcement tells us otherwise) aparently stem from a 2011-2012 catalogue from Walt Disney Pictures. The catalogue aparently claims that the film will follow Mike and Sulley back when they were both studying at the Monstropolis University of Fear, and that they were indeed rivals.
Whilst I was really looking forward to a Monsters, Inc. sequel, to see where it would take the story and how emotional it would be - playing off the nostalgia factor, like Toy Story 3, a prequel sounds like a really original and interesting concept.
Also of note is the fact that Monsters, Inc. 2 is now scheduled to be released on 2nd November, 2012, exactly 11 years after the original film.