I think the iTunes comparison is that it has DRM and you can only print one of each item. I think it's a little early for this kind of service though. The ones who like to tinker with 3D printers will be lukewarm to the DRM stuff, while the ones who just want to buy things will get superior prints through Shapeways.
I would like to add a big BUT though - if home or even in-store 3D printers take off and "just works" I believe there will be a huge opportunity for a marketplace like this. So maybe this will be a prime buy candidate for Makerbot/3D systems or Shapeways some time in the future.
