I think we'll see something developed that will allow you to run Windows apps under Mac OS X (x86) in much the same way that we run Classic apps under Mac OS X (PowerPC) now. I don't think it will come from Apple, though. I don't really see how Apple could lock this down. In fact, I think they should add it to the OS and tightly integrate it so that the line between PC and Mac is gone for Mac users. This would leave, essentially, nothing for anyone to complain about when switching to Mac.
I think you have more to worry about from Microsoft on this one. If they do anything, they will develop VirtualPC for the new platform, and keep you using Windows, in a window, to run your apps. This isn't the same as having real support for Windows apps, but it keeps people using their OS. My idea would remove the middle man (Microsoft) and just allow the apps to run. MS could still make money, too. They could license the right to do this directly to Apple.
I think it will happen, one way or another. The question is, are MS and Apple going to be a part of it? Also, how good will it be if they are not a part of it?