I have a similar issue. I know what my username, email and password combination is for both the forums and the game, but I've owned the game for about two weeks now and still have not been able to log in in-game. I just tried the password reset function on the website, but I haven't gotten a confirmation email yet. This is really frustrating since I can get steam achievements, but I can't post on the leaderboards until this is fixed.
I've tried re-downloading the game and verifying the cache. Every time I try and verify the steam cache it re-downloads anywhere from 1 to 8 files. Every time. This is becoming quite frustrating, but any help is appreciated.