Low-power, low-cost devices have become popular, finding a niche among extremely powerful multi-core machines. The lack of RAM, GPU, and CPU power limit how well Vista functions on these machines to the point where forcing users to upgrade just isn't an option.
But will they do this for XP Pro? While it seemed unlikely in months prior, it seems like a possibility now. Let's also remember than XP SP3 is slated for release in the next few months. Microsoft's original plans were for so-called mainstream PCs to all carry Vista starting this June.