What was seen just prior to Vista's release was the automatic upgrade option seen with a disk included on prebuilds where you could go for an inplace upgrade by download using the extra disk that came with a system. That disk was tied to the one machine using the original factory installation's product key without fuss.
For ordering a disk separately you would have to expect a separate number for that. When MS says go through the company being one of the prebuilders the product key would likely be the same since you would be receiving a custom made branded disk bound to that one machine for that one purpose only.
With a prebuild you have already prepaid the fee for using the MS software namely Windows. The low cost for a disk still being able to use the original product key saves the time of seeing a dealer replace XP with Vista or Vista 32 with Vista 64. What others are talking about elsewhere probably has to do with something else.