Free PC download from Sony.
http://www.jp.playstation.com/psp/dl/tool/data/Custom_Theme.zip

Details:
http://kotaku.com/gaming/diy/sony-release-official-psp-theme-creator-305910.php

Thanks for that info. I might actually pick up my PSP again and try this out. Have you tried it?

Not yet. If I think up any good theme ideas I'll definitely give it a whirl.

Now they just need to make this for the PS3, and I'll be happy. :)

Something like this for the PS3 would be even better. Let's hope Sony keeps opening up like this.