Last I heard, the state of Linux drivers for hardware video decoding did not make it worth it to Adobe to support it. But that news is about 1.5 years old, so maybe it changed in the meantime.
Edit: Flash 11 youtube 360p is using 14% of my CPU. mplayer 480p uses 4%. shrug
Edit: Flash 11 youtube 360p is using 14% of my CPU. mplayer 480p uses 4%. shrug