I’ve moved a couple of my blogs from Serendipity to WordPress (mostly because I’ve discovered that Serendipity has some issues with newer PHP setups, but also because I’m getting lazy and WordPress’ admin panel is nicer than s9y’s ;-)).
Luckily someone else did already write a s9y->WP importer plugin. Sadly it didn’t support nested categories, so I changed a couple of things and here’s my Serendipity (S9Y) importer for WordPress 1.5 (hoping that noone else has already used that version number… the history of the plugin contains several authors :))
One snag: after the import, you’ll need to edit one of the categories (just open and save it) to fix the hierarchy. I don’t know why and honestly I don’t care 😛 Worked for me, your mileage might vary.
Refer to this article for other useful migration tips.