they've split. The wiki entry on the way the split occured is hilarious:

On February 1, 2010, bassist Pete Wentz said on Twitter, "Don't you get it? A hiatus is forever until you get lonely or old. I don't plan on either." "I can't imagine playing in FOB again." "Something would have to change in my head or my heart ... not my wallet."
On February 2, 2010, Spin Magazine released an interview they had with lead singer Patrick Stump, who replied to Pete Wentz's tweets by saying, "I'm not in Fall Out Boy right now. But one way or another, the band will always be around...Whether we play again or not, I don't know. If we do, it will be for the right reasons. If we don't, it will also be for the right reasons." Stump also told Spin.com that he was focusing on his upcoming solo career.

Later that day, drummer Andy Hurley simply tweeted, "I quit too." as guitarist Joe Trohman tweeted in reply, "fine, i quit too."

At the time of writing it cannot be confirmed either way if the drummer wants his ball back.

The only one of the Milkshake gang who actually seems halfway sincere, and who doesn't do that irritating smile-while-tilting-the-head-to-one-side thing when introducing a programme.

Jeez, I can't believe I just posted that.