mplayer supports like... everything natively. So no worries about that. As to how user-friendly the interface is, well, that's potentially an issue. Geexbox is okay but it's too wonky. I think Wii Media is probably going to come out to be the best of this, judging by the early featureset given by the developer.

DVD support isn't a big deal to me though tbh. Using the Wii as a media center for streaming off my PC/USB drive is pretty awesome though.