Movable Type supports multiple Text Formatters, which convert the entries that you type in to HTML. It comes with a simple one that turns blank lines into <p> tags, and you can add others, like Markdown and Textile, which enable you to use a Wiki-like syntax to add markup to your entries when you’re typing them in.
I’ve installed the Markdown plug-in, and the first thing I discovered when I typed a weblog entry and told Movable Type that it was formatted using Markdown was that my dynamically generated pages wouldn’t work because I hadn’t installed the PHP version of markdown. What I find interesting about this is that Movable Type transforms your entries when they’re presented rather than when they’re saved.
I think my first instinct would have been to do the opposite and add the HTML when the entries are saved. The way it works currently, you’re married to those text formatters. If I want to pull my entries out of the database and use them in some other way, I have to look up the text formatter that’s being used and then run that transformation on my text to turn it into HTML. On the other hand, by transforming your text only on presentation, the tool preserves it the way it was entered so that you can easily go back and edit without wrangling with the HTML that was inserted. Interesting design decision, but upon reflection I think I agree with it. Most people want their input preserved more than they want to futz with the contents of their Movable Type database.