I noted also this:
2009-06-19 19:43:38.517 nVidiaVideoSync: VBlank ioctl did not work, unimplemented in this driver?
2009-06-19 19:43:38.517 DRMVideoSync: Could not open device /dev/dri/card0, Nessun file o directory
in the mythtv playback log; is it important? (the wording "Nessun file o directory" means "No file or directory")