Soldato
According to 'Slashgear', "... only customers who purchased the device directly from either ASUS or Google are eligible."
I got the 10% code and then a few minutes later the email asking for my receipt. This news has spread very quickly so Asus have probably crashed , look at their website I doubt it gets more than 10 people a day usually!
I don't know if this has already been posted but if you are having trouble registering use your the warranty starting point as your date of purchase (use http://asus-serwis.pl/info/warranty for warranty). Also I can confirm other retailers work, I got mine from a competitor and received my code last night.
That link comes up in Polish!
I still can't even get it to recognise my serial (SSN or CSSN) number!