I haven't hard wired my PS3 yet... I wonder if that plays into consideration, I'll be hard wiring it tomorrow... we'll see if i get the code shortly after.
Another theory I was bouncing around is that there were only so many codes, and since people that did not receive an e-mail invite were still signing up through the URL they may have taken the spots from those who did get invites.
Then again like above it could just be rolling out in packs... although I think 3 weeks is a bit much for a follow up e-mail.