I saw this on the Inquirer first and was sceptical. But I tracked down the relevant licence terms and it looks like it's true. If you buy the retail version of Vista, you can transfer it to another PC once only. After that it looks like you'll need to buy another copy.
Here are the relevant licence terms for comparison:
XP SP2 Home:
"14. SOFTWARE TRANSFER. Internal. You may move the Software to a different Workstation Computer. After the transfer, you must completely remove the Software from the former Workstation Computer."
Vista Home Premium:
"15. REASSIGN TO ANOTHER DEVICE.
a. Software Other than Windows Anytime Upgrade. The first user of the software may reassign the license to another device one time. If you reassign the license, that other device becomes the “licensed device.”"
(emphasis added)
UH, WTF?
The whole point of buying a retail licence is that you can transfer it to a new computer when you upgrade. This is making me seriously doubt whether I'll buy Vista.