Much has been made of the market share recently scooped up by netbooks, most of which run Windows Vista, but market share can be a deceiving metric. Treating all sales as being equal is how we come by the claim that Apple has a 25% share of the U.S. music market, despite that the transactions it conducts are mainly one song at a time, whereas Apple's competitors mainly sell albums, at 10X to 20X the revenue per transaction.
However, there is one sense in which "a sale is a sale" applies, which is that each netbook sale represents one person paired with one machine, accumulating experience (and perhaps frustration) with the capabilities and limitations of the hardware and the software platform running upon it, neither of which works to the advantage of the other. Aside from being eminently transportable, netbooks are underpowered for a system like Vista, which really wants several times the power of a netbook. For their part, they don't get the opportunity to shine like they might running a more efficient system. The user experiences their netbook constrained by the performance it can manage while supporting Windows, and they experience Windows constrained by the limited hardware.
Also, because netbooks are so inexpensive, the user doesn't have a lot of money tied up in the decision to buy one, so cutting their loses may seem like more of an option than it would had they spent a couple of thousand dollars on a notebook.
The bottom line is that this is a situation likely to produce a new wave of Mac converts, when the users of all those netbooks decide that they've gotten their money's worth out of them, or that they can't afford to keep using a machine that so impedes their own productivity. Having experienced first hand what you get for rock bottom prices, they'll be ready to pay a little more for something better, and many of those who walk through the door of an Apple store to see what Macs are all about will be hooked.
One thing seems certain, the purchase of a Windows netbook isn't likely to produce the same sort of customer loyalty that the purchase of a MacBook does, and netbook vendors shouldn't count on repeat sales.