I just upgraded WordPress to 1.5.1.3, and it went pretty smoothly.
Except it broke my categories (edit: looks like permalinks in general). When you click one, you get ye old 404.
I’ll have to dig around to find out how to fix it.
update: How very odd. After deleting everything and starting from scratch, I simply restored my .htaccess file (which I did not delete), and everything’s back to normal.