The “loophole” discovered in the upgrade process for Vista appears that it will go unpatched in
future releases of the OS, according to Microsoft. The loophole allows someone to install straight from an upgrade disc of Vista, even without a valid copy of Windows XP. Though Microsoft states that doing such violates the terms of the license, the “small percentage” of people that will use such a workaround isn't enough for them to really bother. Then again, they might be able to release a silent software patch in the future that disables said installs, though that is unlikely.
is probably correct. Microsoft is probably more worried about rampant piraters of Office 2007 and Vista than they are about someone getting a small discount on the price of a single license.