I've been working on this for a while, and gotten interest from several people, so I'm posting an announcement here. Building on the work of John Long's multiple controller view paths in Rails core, I've created a plugin for customizing templates on a per-site basis for a single Rails app.
I also want to credit Matt McCray (and Tobias Lütke in turn) for his Theme Support plugin which was my inspiration, and how I accomplished similar goals in older versions of Rails (1.2.x and earlier)
This plugin does absolutely require edge Rails, but it's also a very very clean design. If you need to do something similar on Rails 1.2.3, I have a patched version of Theme Support that I am also planning on writing up at some point.
Anyway, the writeup and project link is here:
http://darwinweb.net/article/Announcing_Multisite_Plugin_For_Rails
