I am not sure if my problem is really comparable to that problem there.
I am not running any window manager, just plain X and XBMC running on top of that.
What I've seen is that "nvidia-settings -q all" will show up only one performance level, 0, and no other levels are possible.
I redirect the GUI of nvidia-settings to a different X server while running XBMC/VDPAU.
Only level 0 is shown at a frequency of 500 MHz.
Once again: I don't need any GL application, like glxgears, to run to have smooth VDPAU playback.
It's sufficient to have something CPU intensive running, like burnK7 etc. The clock rate of the CPU needs to be at least at 1.8GHz to have it smooth. 100% at 1GHz won't do the trick.
So I think this problem must be related to some timing stuff.