If you read this article by David Pogue of the New York Times, you may be inclined to believe that the iphone + app store via itunes model that is being proposes is going to be HUGE.
I do not buy into the hype, in that the proposal is sounding pretty restrictive in that apparently 3rd-party apps will be restricted from multi-tasking. Thus, if you are using an app and then need to answer a phone call, text message, or email, you'll actually exit & shut down the 3rd-party app...this is basically the same thing that any piece of software on the Palm platform must do (and that is an OS that was created in 2002) - my point being that this is an old-school model of doing things.
Not that I'm a Microsoft fanboy, but comparatively, with Windows mobile, one can have 5 things running at the same time, which is actually useful - you could be downloading something, writing a text message, and talking on the phone (via bluetooth) at the same time.
I guess I'll likely not be getting a new iphone anytime soon (although the 3g version coming this summer is appealing since it will be shiny, pretty, and hopefully have fast data downloads)...