I've been playing with this some more tonight, and I'm still confused. I have a second, much smaller (20 posts) MT journal, so I've been testing with that. I've found that if I change import-mt.php to start with step=1 by default, then I actually get some of that page, dying somewhere around the part that produces the drop-down of potential authors. If I replace that whole select with a regular input field appropriately named, then I get the opportunity to get to the next page, which will then limp through an import, complaining about numerous SQL errors, like:
Warning: in_array() [function.in-array]: Wrong datatype for second argument in /mnt/big/www/lesser-evil-test/docs/wp-admin/import-mt-hacked.php on line 166
Warning: Cannot use a scalar value as an array in /mnt/big/www/lesser-evil-test/docs/wp-admin/import-mt-hacked.php on line 168
Warning: Illegal offset type in unset in /mnt/big/www/lesser-evil-test/docs/wp-includes/wp-db.php on line 200
WordPress database error: [You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'AND category_id = 1' at line 1]
SELECT * FROM wp_post2cat WHERE post_id = AND category_id = 1
It seems that there's a problem with uncategorised posts, at least... I still don't see how any of this would make php coredump though.