Bootcamp, virtualization, yada yada yada
Microsoft makes tons of money and has legal headaches. Apple makes not quite so many tons of money and has smaller legal headaches. Here's an interesting possible direction...
Apple makes Windows XP/Vista "the new classic" via strong virtualization (i.e. virtualization where the virtual machine can actually "see" some of the more useful hardware, which is to say the GPU) within OS X 10.5.
OS X 10.6 with Windows Vista bundled (a la Classic) replaces Windows as both Microsoft's and Apple's OS. Microsoft still makes a ton of money from OS X 10.6 (via sales of Office and cross-licensing). Apple gets access to Microsoft's DRM. Windows users get a relatively robust OS. Users get a single OS that can run Windows, Macintosh, and UNIX software seamlessly, play media from anywhere. Apple will lose hardware sales but gain huge market share. Everyone is happy.
Note that Apple is heading this way regardless (and, indeed, Apple has no choice; virtualization is already here and stronger virtualization is the logical, obvious next step).
So it's merely a matter of whether the two companies cooperate to make everyone's lives easier, or insist on creating incompatibilities to force some users to choose one platform and live with the inconvenience and other users to work across both platforms and live with a different level of inconvenience.