Many thanks to everyone that's replied to this issue. I'm glad it wasn't just me.
It's good to see so many people realise that the snapshot generated from an uploaded model needs to be a "proof", even if that is merely to confirm to the person about to spend a hundred dollars getting this printed, that he's not going to get something turned inside out by an uploading glitch!
I can totally understand the situation Shapeways is in, some say smoothed, some say unsmoothed. You can't please everyone.
But i would argue that anyone that wishes to see a smoothed preview, needs to be educated properly about Rapid Prototyping, and gently informed of the reality of what their printout will look like.
If they are unhappy with the faceted reality, perhaps this isn't the method of production for them? Perhaps it will just encourage them to increase the mesh density and face count.
I would hate for anyone to feel mislead by the Preview images, and disappointed with their order.
Wormwood - with relation to your suggestion of a "VPC" - i'm really not sure that sort of information would be that useful. If i take a flowing, curved model, and print it at 1cm with 50,000 faces, it will appear very smooth. If i then scale exactly the same model to 10cm, it will not seem as smooth. So the system you're suggesting appears to work in this situation.
However, if i then design something that is extremely flat sided, with only 1 curved surface, then increases in mesh density become much less related to it's "overall quality".
The information would be interesting to have, but with no way to predict how curved or flat faced users models are meant to be, Shapeways cannot offer a guide number to aim for. Without a guide, the numbers don't really help.
It's great that with print - you can be told "300dpi for print quality". However a cube mesh with only 6 faces may look "higher quality" than a 25,000 face "monster head", even though the "VPC" is telling the user the exact opposite should be true. It doesn't matter if i scale it to 1cm or 100m, a cube requires no additional vertices to retain it's "quality".
I think this is why no one has come up with a 3D version of DPI, apart from a face-count.