http://blog.wired.com/monkeybites/2008/04/microsoft-wont.html

Extracted:
But if you look carefully at Dell’s plan you’ll see why Microsoft is unlikely to find that customers are clamoring for XP. Microsoft offers what it calls “downgrade rights” which allow anyone purchasing a Windows Vista Business or Ultimate license to downgrade to Windows XP Professional.

The downgrade option comes in the form of an XP image CD, which is somewhat different than an actual install CD. Dell is offering to do a factory install of XP from the image CD, which is a nice touch, but you’re still buying the rights to Vista and then downgrading.

Thus, when Microsoft shows its sales reports, it looks like you bought a copy of Vista. Even if Dell reports the number of downgrading customers, all Microsoft has to do is point to the sales records to show that Vista is the customer favorite.