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
e-credibility: the non-guaranteeable likelihood that the electronic data you're seeing is genuine rather than somebody's made-up crap. - Karl Lehenbauer