I’ve never come across a situation where I would need this feature, but I’d like to have this on record in case it ever comes up. Mike has developed a WordPress plugin, Themed Categories that allows the user to select different uploaded themes for different categories. According to the comments, his version doesn’t work with WordPress version 2.3, so someone named Kalle made the necessary changes and the modified plugin can be downloaded here (direct link to zip download).

WordPress Themed Categories Plugin
WordPress Themed Categories Plugin for 2.3 (direct link to zip download)