Rails Theme Support Plugin
Update: I changed the svn URL to point to it's new home!
Currently, to add support for themes into your rails application, you have to install the theme_generator. The generator will then create the folder structure for themes, and it also creates the plugin files for theme support in rails.
The more I've thought about it, the less I like this approach. I'd rather have a fully encapsulated plugin that would add support for themes into rails and define a simple generator that would merely create the boilerplate theme structure.
To that end, I present the Rail Theme Support plugin. To install the plugin execute the following in the root of your application (requires rails 0.14.3+):
./script/plugin install http://mattmccray.com/svn/rails/plugins/theme_support
In the next couple of days I'll have a trac installation up and running for it too.
You can, of course, still get the theme_generator from RubyForge. But I think the plugin will be the approach I will recommend moving forward.
Try it out and let me know what you think!

