Edit: Actually, Popwire is a Universal Binary now. It's a codec for Quicktime that plays all .wmv (Windows Media Player) files, even those that require Windows Media Player 9. You can download it
here. You have to pay for it though; the unlicensed version only supports files up to 20 seconds long. So, er, if you really want to be able to play .wmv files, this is your option right now.