As I understand the agreement, and I'm very far from knowledgeable on such matters, Disney actually could open a Marvel character attraction in Walt Disney World tomorrow, but it would have to be based upon a character which Universal either isn't using, or else is using in only an incidental way (probably something like a character found only in attraction signs or murals or similar; Walk around characters may be incidental if they aren't used in promotional materials (or something like that - again, I'm not an expert at all!). The attraction could also not even be labeled a Marvel attraction, only the characters name could be used (Universal retaining use of the Marvel name). Disney could not have a Marvel land, for instance, but possibly could build an attraction called Machine Man, again, assuming Machine Man isn't used by Universal or else used only incidentally. Disney would also be restricted in how it could market the Marvel character attraction within X miles of Universal.
The agreement is posted on the SEC website (found by members of another discussion board), but I don't know for certain I've found the complete agreement. I had always assumed Disney could always open a Marvel restaurant or retail presence in Downtown Disney (Universal having theme-park rights), but having read the document(s) on the SEC, I'm not sure anymore (the agreement spells out restrictions on a Marvel retail project). Regardless, such a move would make excellent free advertising - for Islands of Adventure, and it's solely my opinion, but it seems like a Marvel based attraction in WDW at this point could be a legal confrontation waiting to happen.