This has been innevitable for at least a year now, but it's still a pretty cool new checkpoint for the "this is really happening" cycle to cross, at last: As of yesterday, the still-in-pre-production "Avengers" movie has officially transfered it's "rights" from Paramount to Disney.
Disney bought Marvel Comics with a plan in mind: They wanted an equally-massive "boy brand" to compliment their massive "girl brand" re: the "Disney Princess" franchise(s), and buying a few thousand superheroes all at once made a certain amount of sense - except it came with a "catch"...
Between the late-1990s and 2009 when the Disney deal was struck, Marvel had already spread-out the FILM rights to a lot of it's "A" characters among multiple studios - so even though Disney now owns, say, Spider-Man... they don't have the rights to make MOVIES about him.
Strictly in terms of the film-rights, it breaks down roughly like this: Sony Pictures owns "Spider-Man," Fox owns "X-Men," "Daredevil," "Ghost Rider," "Fantastic Four" and various ancilliary characters; while Marvel Studios and Paramount had a joint-agreement on the Avengers-connected franchises (Iron Man, Hulk, Captain America, Thor, Nick Fury and the rest.)
Under the terms of this deal, when "Avengers" and "Iron Man 3" come out in 2012/2013 (and their hoped-for sequels after that), they'll come out under the Disney banner - Paramount will retain the rights to "Captain America" and "Thor," both still releasing in 2011 (but if they're both hits, look for Disney to buy THEM "back," too.)
For fans, the immediate-takeaway here is overwhelmingly positive: "Avengers," even before the Disney deal, was meant to serve as the "hub" of the shared-universe-continuity Marvel is hoping to finally bring from the comic world to the film world... and now, there's NO extra red-tape keeping it from interacting with the VAST majority of characters and properties owned by Disney/Marvel. If they want, say, Luke Cage or Doctor Strange or Black Panther or whoever to show up in a big "Avengers" movie to promote THEIR feature; it can happen easily.