I just upgraded wordpress but I was also occasionally having this problem with 2.7 as well...
when I get the log-in screen I type "admin" and my password and literally nothing happens, no "log-in failure" or "incorrect username/password", nothing; the screen just seems to reload its self, the stuff I typed disappears and nothing happens. If I go to the lost password auto emailer from there the same thing happens, it says type in email, so I do, the screen blinks and nothing happens and I get no email.
All the permissions seem to be in order, I simply can't log-in!
Please help!! :^/