He asked the rest of the band if they could take a hiatus from Dream Theater a couple/few years. The other members didn't want to so he decided to leave and let them continue with the band. Mike says it had nothing to do with Avenged Sevenfold, but rather the constant write > tour > repeat cycle burning him out. According to James LaBrie, (and I think Petrucci said this in his forums) there are 7 top drummers who are going to audition soon, and the band is confident they can all perform well, so they're mostly looking for the one with the right personality and look. Still planning on hitting the studio in January.
Mike might be back eventually, but it's hard to see right now. If I were a betting man, I'd say he'd return after the next album and tour.
On a related note, Portnoy's being a big baby about everything and has been deleting threads in his forums that discuss anything related to him leaving or the new drummer. He also deleted a thread talking about James LaBrie's new album because he read a headline that said JLB isn't sad about Mike leaving, though he really said in the radio interview that he's not sad anymore and he's looking forward to the future (or something like that)