I recently upgraded to the lastest WordPress 2.6.1 (From 2.5). One thing that I have to say is when the upgrade is complete, it may appear that you can’t login to WordPress. Don’t go requesting a new password. Instead, clear out your browser cookies and try again.
The instructions recommend clearing cookies before attempting to log back in. I myself reset the password about 5 times before I actually read the detailed instructions on what to do aftewards. I hope this helps somewhere.