Sony has released a tiny application that allows you to create your own themes. Although the instructions provided on the page are in Japanese, the program actually runs in English. It's not a very intuitive GUI, but it'll allow theme artists-to-be the tools they need to get started. Granted, it's a lot of work -- there are a surprising number of buttons on an XMB menu.
Get to work on your theme -- we'll love to see your creations. Who knows? Maybe we'll give away a prize for the best one ...
[Thanks to everyone that sent this in!]
A great tutorial has been made by reader Jay Lee. Check it out