nVidia just released the drivers 185.18.14 ; they are release-candidate status.

They fix quite a fair amount of problems:

1. -With Powermizer active, the speed of the video card wouldn’t get adjusted when playing a video with VDPAU, resulting in poor playback quality.
   

2. -No need for disabling Composite in X configuration file xorg.conf
   

So build mythtv, mythplugins and mplayer against those new drivers. Make sure you read the nvidia drivers entry ; as user action is required