Surely if (and when) Apple gets an x% market share then Windows, would crumble as the software compatability problems would start to go away increasing the market share even more into an eventual snowball.* I'm not sure whether this would also help snowball Linux's market share at the same time.
The real question is what is x? I'd say probably about 15% as Firefox's market share is now about 10% and most websites work properly with it, but not all.
If I wasn't a Mac fan and didn't prefer OS X to Windows and also didn't desperately need a new computer now, I'd wait for Vista to see how bad it is.
If it's worse than Tiger (in most reviews, like Tiger vs XP now.) I'd definitely get a Mac, if it's about as good as Tiger, I'd probably still get a Mac, if it's better (in most reviews like Tiger vs XP now.) I'd probably get a Windows PC.
*As its better!