WordPress 2.6 won't let me log back in. Well, that's what I encountered immediately after upgrading. I instantly deleted the cookies for my domain. Then WordPress let me in. I use Opera 9.51, so it's easy for me to handle cookies on a domain-by-domain basis. I just had to right click on the log-in page and go to "Edit Site Preferences," then select the "Cookies" tab, then select the cookies and hit delete until they were all gone. I could have looked for the offending cookie, but I didn't want to take the time.
Now I'm posting this for all the WordPress users who upgrade and may otherwise panic. It is a sinking feeling.
As for IE, Firefox, Safari, etc., users, you'll have to read your browser's help file regarding removing cookies for your WordPress domain.
If this helps you, let me know. Leave a comment.