There's now an 'Export Wiki to HTML' command. This will make a directory of linked HTML files, and assumes you're using Markdown for formatting (and because Markdown is so excellently unobtrusive, you'll never notice this change if you don't use the export function). A few people asked for this, so I hope it helps. Thanks Fabrice Luraine for showing me how to inherit from the Markdown language.
There's a 'Create New Wiki' command which simply prompts for a directory and makes an IndexPage.txt file there. It seemed easier somehow.
Download here. I'm new to both Ruby and TextMate bundles... so, as ever, patches welcome!