Under Vista, older versions of Windows will run as a subsystem, thanks to the use of Virtual PC Express, a feature of Microsoft's new Enterprise and Ultimate versions. Virtual PC Express allows users to run any previous version of the Windows operating system, and customers who have annuity agreements with Microsoft will be able to install an older version of Windows on top of Virtual PC Express without having to acquire an additional license.
What can be inferred from that statement is that some users who purchased retail editions of Windows XP, or found it pre-installed on their new PCs, may not be able to install XP into the Virtual PC Express subsystem without purchasing an additional license. Currently, most retail editions of XP require some form of product activation, in which a product key is checked against a database of installed systems at Microsoft, to make certain the OS isn't installed on more PCs than the license permits. A user can install one copy of Windows XP on the same PC multiple times, but not on two PCs.