Microsoft is introducing small but significant modification to the Windows Vista End-User License Agreement (EULA) for Windows Anytime Upgrade. Customers who purchase a retail copy of Windows Vista and then upgrade to another version of Windows Vista using Windows Anytime Upgrade will be affected by this modification, while all other WAU licensing terms remain unchanged.
Now, those customers will be able to uninstall their upgraded copy of Windows Vista and re-install it on another device (usually, but not always, a PC). The number of device-to-device reassignments is no longer limited, provided that Windows Vista has been uninstalled from the original device.
The full text of the change can be found at this FAQ under the final question, "Am I allowed to transfer my software from one device to another when I upgrade using Windows Anytime Upgrade?"