The only time I've ever had a brand new car (company car Mundaneo) it had 40 something miles on, while my work colleagues car had 6. I guess it does depend on whether it's been carried from the production line to your door, or whether it's been driven by one of those blokes you normally see standing on the road with an oddly shaped forehead. I was once told that it's some sort of numberplate to say that they're legit, but it always seems to be on the top of their head....
Some cars have the ability to reset the clock once, I think it's up to 100 miles or so?