Never. Apple does not want to play in the low-margin market, so that will be left to others. Figure 30% market share for desktops is the most reasonable target, but all on the upper 50% of desktops by price. Probably higher market share for laptops, but again, all in the upper half of systems.
The rest of the market.... well, that's Microsoft for now, but don't be too surprised some years down the road if Linux becomes standardized enough to be the OS of choice for low-end hardware such as netbooks, because the price of the OS for something like Windows will be prohibitive compared to hardware cost.
To head that off, Microsoft will have to cut prices and deal with smaller profits on a smaller piece of the overall pie, though they could still easily have a majority of marketshare, but without the profits they're accustomed to.
your comments have been made with every windows upgrade since windows 3.1