A little while ago I was looking for a solution to automatically insert Technorati tags into posts and RSS feeds on a Movable Type blog. I found a simple MovableType plugin called MTKeyWordList that can be used for this purpose.

The plugin lets you split the keyword field into a list and then use each keyword using the MTKeyWord tag.
So for example

<MTKeyWordList>
  <dc:subject><$MTKeyWord$></dc:subject>
</MTKeyWordList>

Could be used to add the keywords as categories in your feed.

That plugin can be found at LaughingMeme. It works great, but only works with static pages. The archive pages in the blogs I wanted to add this to are generated dynamically so I wrote a dynamic version of the plugin. It works the same way as the static page version.

If you want to use it you need the following files:
function.MTKeyWord.php
block.MTKeyWordList.php
Just drop them in the php/lib directory of your Movable Type directory.