I spent some of today creating a new plugin that uses the built in theme preview capabilities in the admin pages to create a front end display of themes found in wp-content.
This plugin currently only works with 2.6. but I am planning on making it 2.5 compatible. There is also a lot of development in store, with the ultimate goal being to have a plugin that allows theme authors to showcase their themes and offer them for download.
The plugin can be downloaded from here: http://epicalex.com/epic-theme-viewer/
And a demo can be found here: http://mysoutham.com/?page_id=13
I'd love people's feedback on this, so that I can improve it, and tidy up the code.