Some things I’ve discovered that are not exactly part of the Drupal documentation:

• Make sure you disable all modules, at all times.  If you have a module enabled that conflicts with the update, your day will suck.
• Make sure you set the default theme to one of the included drupal themes, otherwise you’ll end up with an unnavigable site.
• Make sure you backup your database.  This step is an easy one to skip, but the update.php will change your database, and potentially break your site.
• Double check your backups.  My most recent backup had failed, and when I went to restore from it I was screwed.