iPhone is 1 device on 1 network, Windows Mobile is hundreds of devices across all networks.
"Gartner Q3 2008 data shows iPhone passes Windows Mobile": http://blogs.zdnet.com/cell-phones/?p=354
"Gartner Q3 2008 data shows iPhone passes Windows Mobile": http://blogs.zdnet.com/cell-phones/?p=354
Whilst I understand that you're referring to North America you should consider that the iPhone is sold in a lot of other countries.