10 million units in 1.5 years isn't very high volume for a high profile phone model. However, I think that they're being conservative with their public predictions. I also can't see them keeping only the single model for 1.5 years, nor a US-only distribution.

It wouldn't surprise me if they did 20-30 million units (of all iPhones) by the end of 2008 if they open up sales to the rest of the world by the end of this year. If the price was lower I'm sure they'd do much higher volumes, but it would reduce their iPod sales without question.