As we all know very well, Microsoft has always done the best it can to put a stop to any kind of technologies or businesses that might in any way threaten to lessen dependence on Microsoft and any part of the Windows ecosystem. From software as diverse as Netscape to Java, and Wordperfect to IBM and Lotus, Microsoft has time and again shown that they will do ANYTHING to try to maintain the applications barrier to entry for any competing OS or any competing platform. It is for this reason that they have often retaliated against OEMs who try to offer unapproved non-Microsoft products on their computers.
The success of the recent contest to get Windows running on a Macintosh and the large amount of media coverage it generated shows that there is clearly a huge potential market for machines that can dual-boot Windows and Mac OS X. Imagine how exciting it would be, being able to run all kinds of Windows games alongside applications like Microsoft Office for OS X and Adobe Photoshop for OS X. This could very well help to increase Apple's marketshare, which in turn might help spawn more Mac OS X software development. All this could mean lessened dependence on Windows. I am quite sure the Apple folks are well aware of this, and this is exactly why Microsoft does its best to prohibit installation of any other OS alongside Windows by the OEMs, and this is undoubtedly why Apple cannot offer Windows on their machines directly by themselves.
If history is any guide to us, I believe Microsoft will quickly be evaluating this new competitive threat from Apple, and once they have done so, they will be looking very fast for ways to retaliate against Apple. Undoubtedly, they will start by threatening to shut down the Macintosh Business Unit, and all Office for Mac OS X development. However, this obviously will not be much of a deterrent, because after all, people can now run Office for Windows just as easily on a dual-booting Macintosh. For this reason, Microsoft will probably end up taking additional steps. I suppose they could try to launch a lawsuit against Apple--I do not know offhand what Microsoft will end up suing for, but I am willing to bet that breach of contract would not be out of the picture.
Microsoft could also try to render Windows impossible to run on Macintosh hardware. In fact, this may well be the reason why they suddenly dropped EFI support in Vista. Indeed, the timing of this development is very interesting, since it came just as talk of Windows on an EFI-based x86 Macintosh was starting to heat up.
But this is my prediction, and mark my words: I believe Microsoft WILL soon be looking to retaliate against Apple for their move to support dual-booting with Windows. Microsoft has never tolerated anything that could serve as a stepping stone away from Windows, and it seems unlikely they will make an exception here.