Judging from this interview with Tom Brevoort and Nick Lowe, Avengers vs. X-Men might be more truthfully titled Avengers and X-Men vs. X-Men and Avengers, as Brevoort explains:
There’s nothing that even says that characters — especially characters like Wolverine, Beast and Storm, who just joined the Avengers — will choose one side and remain on it consistently… It’s obviously easiest to see how the X-Men among the Avengers’ ranks might have divided loyalties. What may not be as apparent is how certain members of the Avengers might have greater sympathy with the X-Men’s position, or greater confidence in what Cyclops believes. I tend to think of characters such as Hawkeye, Iron Fist and Luke Cage as guys who’ll naturally root for the oppressed underdog against the majority or “the man,” so there may well be some points where each or any of them need to spot-check what they’re doing and why.
By the same token, the X-Men, as I mentioned earlier, are already fragmented, and this situation may cause them to fragment even further — not everybody in either X-camp is so totally supportive of either Wolverine or Cyclops that they wouldn’t exercise their own judgment and own sense of morals if it came down to it. Even among the Avengers, there may be characters who are taking a sidelong glance at the mutants standing right next to them as the situation gets real, wondering if their loyalties can be completely trusted.
On the one hand: It makes more sense, from a story perspective, that the battle lines aren’t as clear as “This team versus that team.” On the other hand, it does make all the Are You an Avenger? Are You an X-Man? branding (and, kinda, the title of the book itself) seem somewhat ridiculous if Marvel’s own editors are saying that Avengers vs. X-Men is an overly simplistic and not entirely correct way of looking at the conflict two months before the series even begins.