Check out theme_support from Matt McCray (http://mattmccray.com/).
He's been MIA and you will need to patch it to work with Rails 1.2.
The plugin is basically extracted from Typo themes, so it has some
limitations. Basically it lets you override any template from an
application. So you can probably accomplish what you are looking for
just by dropping in a new layout, and creating some wrapper code to
select the theme based on the domain name. I am currently using it
for a major product, and have submitted a couple patches to Matt but
heard nothing back. I'm thinking about creating a derivative plugin
called "multisite" or something that is specifically focused on what