We recently bought 4 OEM copies of WinXP Pro here at work (less than a week ago).
All 4 came with a voucher titled "Upgrade Redemption Form for Upgrade Version of WIndows Vista Business"
The form numbers are unique.
You go online to a certain URL and apparently once you fill in all the information you're given an Order Confirmation Number.
You put that number on the form, send it off and they then send you Vista in return.
It does say you need to provide with this form:
A copy of your receipt from your authorised system builder (in theory if a company is authorised to sell OEM copies of the OS they should be an authorised systems builder, so just include your OcUK receipt for example).