I haven’t done the WordPress to MT migration myself (yet), but I’ve suggested a look at these links that (from what I’ve been told) were very helpful:

http://www.movabletype.org/documentation/administrator/maintenance/import-export.html

http://www.movabletype4.eu/wordpress/migrating-a-largish-site-from-wordpress-to-movabletype-4.html

Not sure you have seen these yet.