I noticed artefacts appearing after watching TV for a while, getting out of mythtv, running the screensaver or watching a video, then they appear. Reboot of the MD fixes that. Nvidia drivers 260 are installed.
I noticed this as well running opengl or vdpau for the Media Director under the Hardware Acceleration option.
Changing this to "auto" resolved the issue. I am running 10.04 using the nVidia graphics on my Zotac ION MD. It uses the GeForce 9400M graphics chip.
After setting the acceleration to "auto" and rebooting the MD, the artifacting went away.
Perhaps this may help. I am running stock 10.04 nVidia drivers, I believe it is 260.