So now that Movable Type supports Tags, I'm going to have to re-architect the way I've done my categories.
I wanted to make this work a little more like Technorati with their tag search, where you can just go type in www.technorati.com/tags/[tag of interest].
With a little searching on the web, I found this page in Movable Type's beta documentation. Paraphrasing here for continuity:
This assumes you're using Apache, and have mod_rewrite installed and working. You need to simply put the following into your document root's .htaccess file:
RewriteEngine on RewriteRule tag/(.+) /mt/mt-search.cgi?tag=$1&blog_id=1
Jay goes on to note that if you're using Movable Type templates to create the .htaccess file, you can use Movable Type's <$MTBlogID$> variable to automate your blog_id variable.