This article in the NY Times says that iPhones account for a huge amount of mobile Web browsing. Looking at stats for my sites, I see the same thing. Roll in the iPod Touch and there's no contest. Various flavors of the Windows Mobile OS combined can't touch the Apple stuff.
I guess the thing that surprises me about this whole iPhone affair is that it's the first time that Apple jumped into a market they not only did no own or have a presence in, but it's a very mature market. They managed to just show up and have impact, which is pretty amazing.