At least in the US, a lot of designers have an iPhone (and so do their friends). I think it's also likely that the iPhone was the first mobile device they have ever used the internet on. It's easy to fall into the trap of thinking that since me and all my friends only use the iPhone everyone else does to.
I don't agree. The iPhone was at least my 4th or 5th device that had some kind of "mobile web" function. Most folks had smartphones, blackberries, etc before they got an iphone if they had any interest in the net. The problem was, the reality of tying to actually _use_ these devices to access web sites was such a horrible UX that you didn't do it unless there was absolutely no other option. In comparison, lots of us will idly web browse on an iPhone while waiting on line at the store, etc -- something that probably would never have happened on a Blackberry.
