So apparently I am really, really, really late on this one (three months is a very long time in the technology world). WebM supported in YouTube has been reported since 19 May 2010. Anyway, this means that YouTube now supports three different video playback formats: flash, H.264, and the aforementioned WebM. As for initial thoughts, I have yet to notice any major differences in playback between H.264 and WebM (flash is just out of the question) on my Quadbox.
Yay, it's WebM!
In other news, the creators (esp. Laura Shigihara and George Fan) behind Plants vs. Zombies are awesome!