When using xbmc with vdpau acceleration enabled, I get a green screen (audio
works), and this appears in the kernel log : 
nouveau E[   PFIFO][0000:03:00.0] DMA_PUSHER - ch 6 [xbmc.bin[22420]] get
0x00200f06c8 put 0x00200f0770 ib_get 0x000000db ib_put 0x000000dc state
0x80000000 (err: INVALID_CMD) push 0x00400040

Sometimes it makes xbmc crash completely.

This is on an nv50 (Chipset: MCP79/MCP7A (NVAC) ) : VGA compatible controller:
NVIDIA Corporation ION VGA (rev b1)

Mesa 10.1
libdrm 2.4.52

It works all right using mplayer.
Just a wild guess : something to do with GL_NV_vdpau_interop?

If you need any more information, I'd be happy to provide it.