I had the same problem 3 weeks ago, with all 3 of my blogs.
I tried every solution suggested, including removing all the plugins, renaming the theme folder, replacing the password in PhpMyAdmin, plus the fixes suggested in the trac files.
Eventually, my hosting service (IXWebhosting) help-desk people reinstalled WordPress (3.1) from scratch for me (I had already done this myself, to no avail), and problem was still there – the login page just keeps on returning, and the ’email new password’ function doesn’t work. So they went into the files, made some adjustments, and all three blogs worked.
Yesterday, having done absolutely nothing to change anything, suddenly I have the same problem back again on one of my blogs: https://flloyd-in-voice.com. The website is available online, but I cannot get past the login page. I have tried resetting the password in PhpMyAdmin, deactivating the plugins, it makes no difference whatsoever.
Since I am clearly not the only person with this problem, for whom the published ‘fixes’ do not work, there must be more to the problem than has been so far addressed. I would also be extremely grateful if anyone can suggest something different.