http://www.chess.com/echess/game.html?id=55202900

I won this game by resignation, apparently, but it looks like I should have won by checkmate - am I missing something? 

He can take the pawn en passant.

http://en.wikipedia.org/wiki/En_passant

en passant

En passant! 

Thanks guys. I have a real blind spot for that. 

Yep, after 23...axb3 24. axb3+ Na4 and 25. Rxa4#, 25. Nc6#, 25. Qb4#, and 25. b4#. The most beautiful, in my opinion, is 25. Qc7#

Also, I believe that 24. a4 works as well, as Black cannot defend.

Oh, you mean right there...No, he can take en passant.

technically it is mate, but not in the current position. after 23. b4+ axb3 24. axb3+ Na4 25. Qc7#