Fox's Head of Marvel Movies, Mark Millar, has announced that the X-Men movies and the upcoming Fantastic Four reboot will in fact share the same cinematic universe. Now, he's quick to point out that there are no actual crossovers planned... so what does this actually mean?
Hilariously, not much. Claiming that Fantastic Four and X-Men share the same universe without them actually sharing the same screen is essentially meaningless. Millar might as well claim that Fox's Marvel-verse is the same as the Avengers universe, and Kick-Ass, and Indiana Jones, and then How to Train Your Dragon, because it's never going to be put to a test (Note: We at io9 vote for My Little Pony, because we really want to see the X-Men ride them into battle).
That said, Fox would be pretty foolish if they didn't have Wolverine or somebody wander by the Fantastic Four movie as a cameo; Millar's claims to the contrary, I'd say that — or maybe a quick Skype call from Professor X to Reed Richards or something — is pretty likely. But what does a shared universe that consists of only the Fantastic Four and the X-Men really mean? Sure, they've crossed paths a few times, but they're hardly on each other's speed-dials. Would it only make their cinematic universe feel emptier in comparison to the Avengers-verse? Or is following in Marvel's movie-verse's footsteps the right way to go? Time will tell, presumably.