Windows Media is very imperfect on Macs. Last I heard, DRM simply doesn't work at all with WM for Mac, and many, many people report problems with WM on Mac, even when it should work.

I get the distinct impression that Micro$oft really doesn't *want* it to work seamlessly.

I think Flash is the best cross-platform option (the % of Windows users that have Flash is something like 80% vs much much smaller for QT) but... you have to also consider that your bandwidth will go up since people can't (without some effort) download your flash videos and may therefore watch them multiple times, increasing bandwidth. On the other hand, they also can't download a ton of videos to watch later, so bandwidth from that is cut down, unless you already have tools in place to prevent that.