OK I've had some time to cogitate on this and I guess the sky isn't going to fall. Apple "will not prevent" people from installing Windows on their Macs, but it seems like it's not going to be that smooth an experience. I expect Apple will use its own motherboard design, and unless Microsoft flat out starts building support for Macs into Windows, it'll probably take some tweaking to boot into Windows.
But what will happen, is you'll have a Virtual PC environment or equivalent that doesn't have to emulate an Intel processor. All it will have to do is translate the Mac's hardware configuration into a standard PC configuration, and run Windows in a window or something. With some more work, Windows apps will run as Mac applications under OS X, with the emulator doing things like translating DirectX calls to OpenGL calls and such.
So what about my scenario where everyone says "just run our Windows version under OS X?" I still think it is going to happen. But not with major software vendors. They'll make the transition. After all, they already have Mac versions. They'll probably be able to share a lot of optimization from the Windows version, so maybe we'll see apps that were not well-optimized for PowerPC get performance boosts.
But many smaller vendors that might have been considering a Mac version may not bother now, or won't update an existing program. The question is, will more software from vendors who were not considering such a version be available by running the Windows version than we lose? I now think that is the case. Let's face it, Virtual PC was only useful if you had no choice. But all that vertical market and specialty software that was never, ever, going to appear on a Mac, running at full speed in a translator under OSX, that is going to be interesting.
Man, this was a rambling entry!