I stumbled upon this site the other day and purchased a Diablo Three special edition box, and I was given a voucher when I received the game via courier saying tell a friend to use voucher code..
When purchasing an Official Xbox 360 Headset and get a pound off the £4.99 price!!
That makes it the cheapest price I can find on the interweb at present!
Dosent specify a specific expiry date but it does say two maximum per email address.