I've purchased 4 new iPods (all at various Apple Retail stores) and no one has ever asked me what platform I was going to use them on. Indeed, if they asked, I would just say they are gifts and the recipients might use them on either PC or Macs.
Basically, this is a number that doesn't matter much. All Apple needs to do is use a little surveying and they should be able to approximate the true numbers with a high level of confidence.
I agree that Windows folks have held back since iTunes hasn't been ported yet. The iPod experience on Windows is dismal when compared to on the Mac. Once this changes, you will see Apple really push this device to Windows users.
But, Apple is still missing the iTunes (and Music Store) side of the equation for Windows folks. Apple would also include the USB cable in the box if they were serious about really courting Windows people right now. iPods aren't cheap and certainly Apple could include a USB cable for not much added cost to them.
I think Apple is waiting for the entire picture to come together for Windows users, before they get too excited about how many Windows users are adopting iPod.
Kevin