"Downgrade" Microsoft Vista to Windows XP?

I never knew this was an option:

Microsoft gives Windows XP six more months

[...] Ever since the release of Windows Vista, Microsoft has offered customers "downgrade rights." Essentially this means that those who purchase Vista Business or Vista Ultimate have the option to use to Windows XP Professional on their PC and then move to Vista when they are ready, without having to pay for an upgrade. OEMs have supported this option forcefully ever since Windows XP expired on June 30, 2008 (XP was no longer licensed to OEMs and retail sales were terminated). Downgrade rights never expire, however, so those who insist on using XP will be able to as long as they can get their hands on Vista.

These "downgrade rights," which are available for almost every release of Windows, are a little more heavily publicized with Microsoft's latest operating system due to the unusually long gap between XP and Vista's release, and the accompanying problems when Microsoft decided to majorly change the driver requirements. [...]

This is the first I've heard of "downgrade rights". I'm going to read up on it more.


UPDATE 10-18-08: Here is a link to more details about it.

How to "downgrade" Windows Vista to XP