Good for Apple, bad for Microsoft
This can only be a good thing for Apple and a bad thing for Microsoft.
First let me say it was inevitable. Even if dual-booting never happened at some point we'll see near-native speed using virtualization. Apple knows this. In fact they filed a patent describing how a user can switch between OSX, Windows, and Linux. Since Apple will not liscence OS X for PCs, their computers become the superset - the buying decision with the least amount of risk.
I'm a recent switcher to Macs from Windows even though I had been admiring OS X since it was first introduced. Like many, it was hard for me to switch because my work required me to use Windows-only programs. Last year's introduction of the mini made it easy for me to test the waters - dual-booting will have the same effect on the thousands of people on the fence.
Although there will be some downside (some Windows programs may not be developed for OSX) I think the upside is far greater. Like someone getting a taste of freedom for the first time, once they take OSX for a test drive they'll be hooked. I know I am.
While Microsoft will be able to sell more copies of Windows. It hurts them that a barrier to switching is crumbling. As an ex-employee of Microsoft I can tell you they spend an emormous amount of effort finding ways to keep your entrails wrapped around the Microsoft technology stack. This is not a good day for my ex-colleagues in Redmond.