The web is rife with articles and blog posts citing statistics of Windows vs. Linux on netbooks. Nearly all of these articles mention the stats of the pre-installed OS from online and brick and mortar stores. A quick search of NewEgg.com yielded one netbook with Linux pre-installed and 63 running Windows XP. There were also another 15 running various versions of Vista. So it seems that those that desire to purchase a netbook running Linux have little choice from retailers.
Sure, some OEMs offer a Linux netbook on their sites. Dell offers a version of their Mini 10v with Ubuntu, but it's somewhat hidden off in a corner of their website. One must purposefully go looking for Open Source PC's in order to find it. HP is at least a little friendlier to the Open Source Community by not burying their Linux offering in the bowels of their website. HP's own Mi Linux distribution could be considered to be handicapped by some, though. Linux specific vendor System 76, of course, proudly proclaims their use of a GNU/Linux based OS.
Even with these offerings from some big OEMs, (and one small), there is a much wider variety of netbooks available that can only be purchased with a version of Microsoft Windows installed. Many netbook buyers have chosen to make their purchase based on the hardware and price with the knowledge that they can install their Linux distro of choice. Reading the MSI Wind forums indicates that this is indeed a poplular choice, while all the market research seems to ignore this segment of netbook users.
Because such a large segment of users has been left out, I have started my own non-scientific poll about netbook operating system usage. Let me know what OS is on your netbook by voting in the poll to the right.