Flash!
"Ah, but have they bothered to make it play flash video in full screen? "
Yes. Flash 9 seemed to really burn CPU cycles to fullscreen, as near as I can tell it was probably scaling within flash and just sending a video that's like 10x by 10x as big to the poor X server. It worked for me, but I can easily see it not working, it was noticeably inefficient even on a 3.0ghz box.
Flash 10? I think it might hook into like ffmpeg and SDL or something (since they're designed to decode video and get it on screen, why reinvent the wheel?). Fullscreen behavior is WAY different, I can tell it is probably sending the original-sized video to X and letting the card scale it, it looks better and plays smoother. It still burns more cycles than mplayer for instance, but it's far better than it was.
So, I don't have any 64-bit systems, and have heard of nspluginwrapper, but am nevertheless pleased that Adobe is putting out 64-bit flash. 64-bit Linux users already have it FAR better than those using 64-bit WIndows especially, but this'll make for the "icing on the cake" if they don't even need 32-bit compat installed eventually.