I've also had this - I got logged off while I was away from my desk, so I don't know how long I was inactive, but when I tried to log in again it said "Already logged in" and wouldn't let me back into the site.
EDIT: Ahh, I see - after entering my login name and password, but BEFORE hitting "login," look at the bottom where it says "Forgot password?" and there are the "kick" and "kick and resign" options. Kicking myself off let me log back in again.
Reading the full list of connection problem questions answered this issue, until it's fixed anyway.