Microsoft has committed the blunder of the century. By introducing Vista, a supposedly revolutionary version of Windows, they are essentially inviting their customer base to consider upgrading both operating system and hardware.
And once you consider upgrading operating system and hardware, you might as well consider a Macintosh! After all, the Macs not only have the reputation for reliability and ease of use that Windows is striving for, but they can now run Windows! So you don't even have to change all your software at once.
Apple has, of course, wasted no time in playing this up in their marketing campaign, inviting PC users to upgrade to a Mac. It certainly sounds tempting.