The UK Microsoft approved wholesalers have been pushing a similar deal for a couple of weeks - except the UK upgrade price is around £15, not $15
Essentially with each Win7 OEM Windows licence will come a certificate allowing an upgrade to Win8 later. The offers out now.
In theory only Microsofts OEM system builder partners can take advantage, and based on previous similar deals they'll want to see some kind of proof of purchase by the end customer of a complete PC from the System Builder- NOT proof of the purchase of a Windows CD / licence.