Well since they won't support an in-place upgrade to Win7 from XP, they must be planning some kind of clean installation scenario. I expect we'll see a CD dance routine where you insert the new, then the old to verify that you are eligible for an upgrade, then re-insert the new to get on with the installation.
An alternative would be some sort of online verification system where you input both your old installation key and the new one then get a special key for clean installation. That would cut down on using a single old license for multiple upgrade licenses.
Win 7's success depends on moving as many users as possible from XP; if the hardware supports it, they want people to move.