Come to think of it, I do remember now when Office 2008 was came out, that folks who did the free upgrade deal from Office 2004 had a similar outcome. You didn't just get a retail copy of Office 2008. Instead, it was an upgrade disk that would only install if Office 2004 was already on the hard drive.
MS has spelled out the upgrade path completely here:
iv recently brought this product didnt get to the best start first of the disk didnt work then i was given the wrong product keys then the new product kes i was given was wrong but now it is all fixed, but i was just wondering when you register for the free upgrade are you meant to recieve a conformation email of not