I read the news that Samuel L. Jackson and Hillary Swank will be playing bit parts in next year's Iron Man movie. Jackson will presumably be playing Nick Fury and I would guess Swank a strong contender for The Black Widow. There have been rumors circulating that the Iron Man movie would lead into spin-offs and crossovers with a larger Marvel movieverse. It would be unprecedented to see something like this happen within the live action comic book movies.
As much as every fanboy would love to see a Michael Clarke Duncan as Kingpin cameo in one of the Spider-Man movies, it's hard to imagine the Daredevil and Spider-Man movie worlds co-existing. Despite X2's nod to the Fantastic Four, the X-Men movies and the FF movies don't really seem to fit in the same world. In fact, every series that Marvel has generated in the last few years seem to each take place in their own world. No shared universe (save Elektra and Daredevil).
That's understandable for three reasons. Each property has largely been overseen by one creator. David Goyer for Blade, Raimi for Spidey, Tim Story for FF, and Singer for the first two X-Men movies. Everyone has their own creative take on their assigned property and those takes are bound to be different. Even greater than that, many of these properties have been acquired by different studios. A Spidey/X-Men crossover would be awesome, but 20th Century Fox and Sony aren't going to join forces on that very easily. Finally, in most cases, the story dictates a certain uniqueness for the title character(s). Spider-Man is one of a kind, not one of many. Within the Spider-Man movie world, a team of mutants that operate just outside New York would certainly cross paths with Spidey at some point. Or if not, Spidey does not go through any grief for maybe being a mutant. That never even comes up. So, rather than address the issue, better to ignore it.
It's this last point that transfers over to other comic book movies. The DC characters haven't shared much better. I can hardly imagine any other comic book character showing up within the Christopher Reeve Superman films. The first movie references that there are several alien races in the universe known to the Kryptonians. Imagine if they were Oans or Thanagarians. Hard to picture that in the world of the Superman movies. Although Batman Forever contains a nod to the existence of Metropolis, it's hard to imagine the Keaton/Kilmer/Clooney Batman co-existing in a universe with Superman. Even harder to think of the Nolan-directed Batman in the same sci-fi universe as Superman or Green Lantern or fantasy world as Wonder Woman.
Now, Iron Man and some subsequent Marvel movies seem to be exploring this idea co-existence. I hope it's successful and doesn't turn into what happened with the Incredible Hulk made-for-TV movies when they tried to introduce Thor and Daredevil. Entertaining movies, but jarring to think that these characters now co-exist. Smallville has had some success with this idea by introducing a set of Justice League characters. But in order to allow these characters to share the universe, not a single character has remained true to his comic book counterpart.
It will be interesting to see how these factors will play out in a potential shared Marvel Movieverse.