I used WP Automatic Upgrade to go from 2.5.1 to 2.6.1 and was no longer able to login on a Godaddy hosted site and Fantastico install, even after a password reset. (Had no problem on other Fantastico installs on other hosting companies.) Thought I would have to reinstall WP but clearing cookies worked & can log in fine now - just passing it on since it was a simple fix.