I had a client with this problem just yesterday. Their situation is pretty unique, but who knows, may help you.
Turns out they had either moved their installation at the same time as upgrading, or had some old forwarding that wasn't working now. Which sounds pretty vague, but here's what it caused.
I was going to http://www.TheirDomain.com/wp-log-in and couldn't log in. And when trying to do the "forgot password" function, it would just refresh instead of sending a re-set.
So I viewed the source, just for the heck of it. And the url info in there was for something like http://www.TheirDomainBefore.com (i.e. something other than what it should have been).
Meaning they had the wrong url info in their settings within WordPress. Therefore I did this fix: http://codex.wordpress.org/Changing_The_Site_URL#Quick_fix_method
And got them in. So maybe view your source just for the heck of it and see if the wrong URL info is in there?
It's worth a shot.
-Sherry