Marvel released this teaser (to the right) on Monday morning, with no other details than the nature of the project being teased would be revealed on Tuesday at 3 p.m. eastern time. As it happens, that's also the time and date of Marvel's latest "Next Big Thing" conference call with the comic book press, so it looks like we'll be finding out some details imminently.
Given that it's the signature Avengers "A" with an X-shaped crosshairs over it, the obvious guess would be some type of Avengers/X-Men interaction. Keep in mind that the teaser is promising something slated for December 2011, which is also the announced debut for the Jeph Loeb and Ed McGuinness "Cable Reborn" series.