I'm sure if you bought from a qualifying retailer, they would give you a card with a unique code to use on that site. AFAIK it's just for European customers.
The site is legit, Noka on Twitter pointed my to it, and it's linked from the Terms and Conditions on the Phones 4 U website. It's only for Phones 4 U customers apparently, but I'm not sure where my unique code is either?!