All else aside, I'm sure that a significant factor has been the fact that you can run software for Windows on an Intel-based mac without emulation overhead.
Another factor that I'm sure hasn't hurt is that after the transition to Intel systems, the macs reached something close to price parity with the Windows OEM's. In some cases, the mac ends up being the less expensive option (specifically, the Xeon-based rigs), which is also new territory for Apple.
We should cross check market share change of Linux Desktop at the same time. I think due to Vista's snafu, both Linux Desktop and OS X gain market shares. But it seems OS X grabs much more than Linux Desktop does.
Personally I find Linux desktops to be extremely frustrating, buggy, inconsistent and not well designed. Many applications are ugly.
If you live in a terminal/firefox world you probably won't notice this though :-)
S.
We often thought marketing doesn't matter to tech product. But I think marketing matters. But even marketing matters, if the product doesn't match expectation, the product will not go anywhere. The best marketing is done by users. When people see others constantly use Windows XP, they use Windows XP. When people see others use iPod, they use it. When people see others use Google, they use Google. It may be a herd effect. But most users don't care the subtlety. They have too much to worry in their life. Gadget, personal computer, Internet are just part of their life.