A client wants to play a video clip on this photo frame (http://www.pandigital.net/search.asp?Mode=Product&TypeID=17&ProductID=18), which supports AVI at 800x600.

I have the clip finished in ProRes at 800x600. I've tried re-wrapping via MPEG-Streamclip using various codecs, but not much luck so far. One option providing good quality with a somewhat large footprint (2GB) is DVCPRO50, and another, providing a much smaller footprint, is MPEG4 (73 MB). I'm using a Public folder on MobileMe for file transfer. It took around 5 hours to upload the 2GB file, which shows up in the folder, despite an error message at the end. The MPEG4 file did not seem to playback well on the PC.

Any suggestions for a reliable codec to get the job done?

Unfortunately, Pandigital does not specify what frame rate the frame prefers but they want the file to be in the PhotoJPEG codec in an AVI file. Perhaps the Apple PhotoJPEG codec that's available in MPEGStreamclip will do the trick. Keep the frame rate 30 or below. Also experiment with a 640 x 480 file as the frame might not be able to play a larger sized video file.