From what I have read it is trivial to enable VDPAU in MythTV and mplayer. MythTV supports VDPAU only thing needed during the compile is --use-vdpau option.
MPlayer needs to be patched and of you go
I don't see why it wouldn't go in LinuxMCE ASAP because it gives s real benefit to users of Nvidia cards that support VDPAU. Is there a way I can help this gets implemented? I'm not a developer but if somebody gives me some tasks I can handle I would like to help.