A lot more info to come at SDCC I'm sure, but from what I understand is that "Uncanny Avengers" is a combo book - not sure if there is two factions (Avengers and X-Men) or if it is some sort of new team with both Avengers and X-Men included. They have a little more detailed information available at EW now:http://popwatch.ew.com/2012/07/03/marvel-now-jean-grey-exclusive/
Here's what the mentioned about Uncanny Avengers:
Remender describes this as “a bridge book, something that can delve into both worlds. It has a natural purpose for existing after AvX, where we can focus on human-mutant relations. The broad strokes are: Captain America comes out of AvX and recognizes that he hasn’t done enough to help the mutants.” The book will be drawn by John Cassaday, and will include heavy-hitters like Cap, Thor, and Wolverine, but Remender is also excited to focus on some less well-known characters, like Cyclops’ little brother Havok. “Havok’s always been the black sheep rock-and-roller of the Summers family. He can’t do that anymore. You’re going to see Havok become one of the biggest players in the Marvel Universe.”
There is a bit more at the link. I think all of these three books (Uncanny Avengers, Avengers, and All New X-Men) are supposed to be in continuity - although the X-Men book (by Bendis) is a bit of a wild card because it involves the original team traveling to the current timeline (a way of bringing Jean Grey back, as well as younger versions of the others). Not sure if the "regular" versions are around as well, or if they are replacing them, or what. Also, I did read that the preview image making the rounds (by Joe Quesada) is not any particular team, more of a promo for the whole "Marvel NOW!" initiative. They are very specific to say it is not a reboot, but a sort of relaunch with books launching from Oct to Feb.