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 you're asking.