In the preview for Avengers vs X-men, they show a 'line-up' of Avengers and X-men. One of the Secret Avengers listed is Beast, an X-man.

This seems a little unexpected; is this something already revealed in the comics, or is it 'to come'?
In the preview for Avengers vs X-men, they show a 'line-up' of Avengers and X-men. One of the Secret Avengers listed is Beast, an X-man.

This seems a little unexpected; is this something already revealed in the comics, or is it 'to come'?
Beast, one of the original X-Men, joined the Avengers in Avengers #137 (July 1975), which is around the time that the second generation of X-Men debuted, which excluded original X-Men Beast, Iceman and Angel. He stayed with the team until issue #211 (September 1981), leaving them to join the Defenders (1985) and X-Factor (1986).
Beast returned to the X-Men in 1991 and stayed with them (as a member of their Blue team, and later in the series New X-Men and Astonishing X-Men) until his departure in Uncanny X-Men #519 (February 2010). Beast left due to disagreeing with how Cyclops was leading the X-Men, namely his militaristic methods and allowance of Magneto on the team.
Beast then joined the Secret Avengers (July 2010), where he is still a regular member. And as a result of the recent Schism storyline, where a larger faction of X-Men led by Wolverine similarly disagreed with Cyclops methods and split off to form a school in Westchester, Beast has returned to the X-Men in the comic Wolverine and the X-Men (December 2011).
Over time in the Marvel Universe, many characters moved from X-men to Avengers and back again. Beast was one them.
The Marvel Universe - Secret Avengers page confirms Beast's membership as a result of it's formation following the Superhuman Registration Act.
There also appears to have been a falling out between Beast and the X-Men over the return of Magneto to their ranks in the Nation X story arc where he leaves the team again (Uncanny X-Men #519). This appears to be the last formal parting of ways from the X-Men.
Of course, given the complexity of the Marvel universe, the Marvel Universe - Beast page doesn't cover this particular story thread, but I think that's incidental.
I certainly cannot elaborate on the most excellent Accepted Answer, but I thought I'd offer some visual aids.
From Avengers #181 (1978), we have Federal watchdog Peter Gyrich dictating the reshuffled team roster. Even under these bureaucratic conditions, the Beast seems pleased to have been kept on the team.
Fast forward to 1998, Avengers #10 (v3) features a celebratory parade in New York City, honouring the Avengers. Part of the fun is a classic George Perez pin-up page of team members thru the years. One furry blue hero is well-displayed.
In the same issue, watching the parade on TV, the X-Men discuss just who has the better claim to Dr. McCoy.