I manage three blogs. The first one (my test blog) upgraded with no issues. The second one resulted in the damned redirect back to the login page loop. I looked here and saw that someone had solved the problem by deleting cookies. I did this (Opera 9.5) and got back in.
Today I upgraded the third blog. Same bloody loop. Aha, says I, I can fix this. I deleted cookies. Same loop. Deleted cookies, cache, restarted Opera. Same loop. Tried Firefox 3.0. Got on without a problem. Went to my second machine, used Opera 9.5. Logged in without a problem! Back to the machine where I need to login. Same &$&#@&* loop. Read some more here. Tried the wp-admin.php?logout trick. Doesn't work.
I tried carefully checking cookies after each step. First I deleted the cookie. I tried to login, got a cookie. Deleted it. Did not work, wound up with a new cookie. Tried the logout fix, checked the cookies: logged in cookie was gone, test cookie still there. Tried to login, same [expletive deleted] loop. Cookie from hell was back.
I'm out of ideas. Clearly this is Opera fighting with WordPress, but I can't tell who's at fault. Thought about hacking wp-login.php but after looking at it, gave up on that idea unless I get really desperate. That could be any time now, this is my wife's blog and she's getting really impatient.