If you have a PSP and aren't using its video playback capabilities other than via UMD, you're missing out on one of its best features. The PSP can basically serve as your personal wide-screen video player, which comes in quite handy for those who travel frequently. A 1GB Memory Stick Pro Duo card can hold several hours of good quality video and still have room for your music, pics, etc.
The only problem with this is that the PSP is very finicky with regard to MP4 files. It'll balk on image sizes, frame rates, bit rates, file names, file locations, and just about anything else you can think of. And to top it all off, each firmware release for the PSP seems to handle things differently. But then again, I suppose if it were a simple standard process that remained consistent, Sony wouldn't be able to market and sell software to manage this process for you (a.k.a. PSP Media Manager).