OS X will not be greater a threat to Linspire or Xandros or the like than it already is, because OS X will never be able to (officially) run on the same machines that Linspire and Xandros run on: Generic x86 machines. And what does that mean for the market?
Well, if you wanna run OS X and you don't already have a Mac, you'll have to buy a Mac. Just like today. If people wanted to run OS X instead of their favorite Linux desktop distro, they would have already bought a Mac. Since they haven't, that's your proof right there that nothing major is going to change.
I'm sure a few (including me) will try to reach triple-boot nirvana after the Intel-based Macs come out, but not most users.
This doesn't fundamentally change anything for the average Linux desktop user.
Steve
There's no sense in being precise when you don't even know what you're talking about. -- John von Neumann