Let's have a little thought game. Let's suppose that when Windows Longhorn comes out, the users don't like it and Apple continues to pick up market share. What impact will this have on the OS and hardware market?
Quality PC makers like HP will be seeing their sales drop because the Mac platform is still tied to Apple hardware, and will be for some time - PearPC notwithstanding. For this reason, they will be looking for a better desktop operating system so they can keep selling their hardware. What gives? Linux. Although Linux does not quite run perfectly on some devices such as laptops and especially tablet PCs yet, PC vendors will be helping out to make it run even on those machines.
That way, they may end up building a desktop operating system that is more advanced than Mac OS X, depending how conservative they are in their choices of versions and how much extra work they are willing to put into QA. RedHat is very well placed with its Fedora distribution, which already covers much of the home desktop market, even in Europe. Debian is still a strong force, and Ubuntu will be the prime champion of the Debian way, because it's desktop, it's good and it's free. Gentoo is a possible choice for high-specs machines, Mandriva are in a hole again, and Slackware and Arch will be lingering in their specialist, Unix flavour niche. Suse causes users too much frustration, but alas, some hardware vendors may be unaware of this and will perish.
An interesting question, again, is Mono (see earlier post). Hardware manufacturers must stay clear of this, otherwise they may find that Microsoft's got them by the balls after all. This is why RedHat's choice to implement huge amounts of Java (which is nearly as good as C# aka Mono) in Fedora is smart and may give them a head start in the desktop race.
Not only will PC vendors potentially push ahead of Mac OS X in terms of technology, but they may find it works out cheaper, too, particularly as driver problems will be fixed in kernel development, rather than hardware vendors having to keep replacing faulty driver software.
The upshot is that Apple will be driving everyone else to become software-hardware vendors rather than pure hardware.