Wordpress to Drupal Migration Script - but it currently seems to only be able to migrate from flat file to flat file - you may need to first set up a proper database as the target.
Update: well, the script really only transfers the posts. No post slugs (so no URL preservation), no categories, nothing. You might be able to use it if you only had a very simple WP blog, but otherwise it's pretty bare-bones. And with large blogs it crashes after a while with an error - the available memory is exhausted. This is because PHP uses the CGI settings when run from the command line - and the memory limit is restricted there too. On top of that, it has no duplicate detection, so it cheerfully imports everything again on the second run.
Sounds like I'd probably need to write my own solution if I wanted to try this seriously.