One thing that's always bothered me about theming SharePoint is that essentially you are modifying system files. While this is unsupported, it's the accepted practice – Heck, the theming Queen herself, Heather Solomon takes you step by step through doing it this way… until now :)

Robin Meure from Zevenseas has come up with a new Central Admin app page that allows you to select a Web Application and essentially "Nominate" the themes you want to be made available to the site collections within the Web Application. You can also add New Themes without modifying the spthemes.xml file (that we all know and love) – From robin's blog post:

The only thing you have to do is add the Theme to the THEMES folder and add your own custom SPThemes.XML somewhere in the 12 hive (a folder in the FEATURES folder could be a good example) and add this path to my solution and you’re done!

You can download the compiled code from here, and see the blog article with usage instructions here.

Thanks Robin!