During a new interview out this week, Marvel's The Avengers director Joss Whedon said that, like many fans, he'd love to see Spider-Man, the X-Men and the rest of Marvel's currently-leased-out film properties integrated into the Marvel Cinematic Universe--but that it would present a challenge and that until and unless he's presented with that challenge, he's happy to make the movies he's making.
Whedon told CBR, "You give me the parameters and I work with them. I have so many actors, I have so many characters to service that it's not like I'm like, 'Well this won't work unless I have Spider-Man and Wolverine!' I'm just making my movie. Yeah, it's cool to think about stuff like that and if we reached an era where those studios could cooperate on that level I think that would be super dope. Whenever you see somebody crossing over to something it gives it pop. Like Murder She Wrote and Magnum [P.I.] -- great crossover. I got excited."
I know what he means--I loved it when Detective Munch showed up on The X-Files, too.