Carlsen vs Shirov (2011): Why did black resign?

I was playing through this game (http://www.chess.com/games/view.html?id=13182095):

And I'm left wondering: why did black resign in the end? What was wrong with Kxd8? Or did he just lose on time?

edit: I wanted to post the diagram in this post, but can't get it working.. 

33. Qb8 34. Ke7 Bb4+ seems to win.

Scottrf was right black loses a rook.

After 33 ... Kxd8, it's mate in four (one of which is a useless Queen dump):

34.Qb8+ Ke7 35.Bb4+ Qd6 36.Bxd6+ Kf7 37.Qf8#

I'm sure someone will correct me if I'm wrong...

After Qb8 black plays Qc8, he loses the a7 rook. I don't think there's a mate.

Well I didn't see that :-D So after 33 .. Kd8 34. Qb8+ Qc8 35. Qxa7 black would be down a bishop and a pawn right? The rook is traded for the white one. I guess with these ratings it wouldn't really make sense for black to keep playing..