Apparently, Beckett suffered an oblique injury during his last bullpen session. The extent of the injury is unknown, but word is that he might not pitch again this season. The Red Sox are making an announcement after the Red Sox/Yankees game.
If true, this is terrible news. I really don't think the Sox can win a playoff series without Beckett. If he is indeed unable to pitch, the playoff rotation would be Lester/Matsuzaka/Wakefield/Byrd. Luckily, Lester will be able to pitch Game 1 on five days rest if needed.
UPDATE: Good news. Beckett indeed suffered a strained oblique, but it was not as severe as originally reported. He will be starting Game 3 of the ALDS. (Per Tito in the press conference)