On the official Youtube channel for Disney Infinity they have started a series of videos answering the question "What is Disney Infinity?". In this first episode they very briefly cover the four announced play set franchises and the characters that form part of them, including confirmation that Mike and Sulley won't be the only characters from Monsters Inc/University that will be playable at launch.
And it seems like they're looking for questions to answer so make sure to ask any of your burning questions over in the video's comments.