I just got my copy of Sins, and I've run into registration problems as well. I'm trying to register it so that I can update the game, and when I try to register my serial it doesn't give any confirmation or errors. It just reloads the same page with the serial number field still filled in and my account doesn't show me as having Sins registered.
Any help? I love the game and have recommended it to all my friends, and I hate to see something so great be marred by something so small.
EDIT: I also entered my Sins serial number when I signed up for an account, and had a problem of my preferred account name already being taken the first time, if that makes any difference. Hopefully I didn't stumble on a glitch that registered my Sins key with the other person's account when I tried to sign up the first time.