Good friend of mine and media center Guru Niall Ginsbourg recently had issues building his Windows Sidebar Gadget on a 64bit computer (but was working on a 32bit machine).   Turned out he was doing an Xbap in an Iframe that played a video.  On the 64bit computer the sound from the video was working but not the display.

Turned out the issue was the software AND hardware configuration his dev machine was running 2 x 7600GT’s powering 3 monitors on my dev machine and Sidebar was running on his 2nd monitor swithing Sidebar to running on his primary monitor.