I was just explaining this to a friend, actually, so it's convenient that you ask! When I decided that I wanted to have some material show up conditionally, I figured that I could write a simple Perl script that I'd invoke from my individual archive files and that script would look at the page, figure out what category it was in, and then output additional HTML in the case of a match.

Problem was, that's a terrible solution because it's hugely processor intensive and given that I serve up 2-3 pages per second on this server, it'd be a significant burden on the system.

Instead, I dug around in the extensive Movable Type Plug-In Library and came across a plugin called Compare. It lets you have conditional code in your templates based on specific equality tests, including testing for what category or categories match the given entry.

Even better, it's all processed once as the page or pages are rebuilt: because Movable Type creates static pages rather than dynamically generating pages out of a database on demand, it's also far more processor efficient, meaning that each and every visitor has a better experience as their requested page is served up faster. What's not to like about that?

Here's how the code inclusion works. First, just the skeleton:

You can see that it's pretty straightforward, if a bit awkward. Basically just think "if a = b" and you'll get what's going on.

The final block of code in my actual individual archive template ends up looking like this:

Hope that helps you get everything working on your own blog.