Jeff and Young at Development Seed have released a module that allows you to specify contexts: http://drupal.org/project/context

This is very similar to a custom approach I've used on MothersClick.

Essentially you can programatically define your contexts and then change your module & theme behavior based on the context. The best example is how I get node teasers to look dramatically different (not only CSS, but different fields shown) based on the context: on a node listing page, within a block, in a node queue, etc.