I tried mplayer2 and both multithreaded playback and vdpau result in smooth playback of full hd video. Nice.
I also tried to update the nv driver via ppa but this did not improve xbmc playback. Actually, it seems that not only
full hd video but also low definition videos suffer a great deal of jitter when played back in xbmc. I did install this
from maverick repository, since official ppa does not yet exist for nutty narwhal, so maybe this is a bug in xbmc.