I exported my MT blog to a text file, then ran wp-admin/import-mt.php to import the entries. The first clue that something was wrong was that import-mp thought there were a dozen authors (there is only one). So I exported again, this time using Internet Explorer, and tried again. The second import looked good, until I noticed that any HTML tags in the old entries were now literals. I found a couple of references to the problem, which was caused in the export. I tried again, using Netscape Navigator to export, but the text file still had all HTML characters converted to entities.
I did about 20 minutes of search-and-replace on the file, then imported successfully, hurray.
There seems to be very little documentation on WordPress; I guess experimentation is in order.