there was a guy who managed to get the MS release of their "vc1 demo decoder" to work with mplayer in order to convert wmv files, though it couldnt play them real time. it's very advanced stuff though.. while i could figure out his procedure for getting mplayer cvs compiled from source i could not get that working.
I still say flip4mac is your best bet. It offers greatest integration and flexibility.. i think since ms started subsidizing them theyve begun offering a low cost end-consumer version, though if you feel like being an evil warez person, there are older and more klunky versions of flip4mac available at convenient p2p nets like amule.