Founders at work is quite bulky whilst an iPhone fits in your pocket. When you've got 10 mins free on the bus you can whip out your phone and read an interview or two.

I read Practical Common Lisp and OnLisp on my Windows Mobile phone while commuting.

Incidentally, being able to manipulate the file system is the biggest advantage I see in WM vs iPhone. At least for geeks.

San Francisco Muni - Standing up with one hand holding an upper bar. Very easy to hold iphone in 1 hand and turn pages with the thumb of the same hand.