I have to say I am becoming a little sick "bright shiny object" syndrome that has struck a large portion of the marketing community regarding iPhone apps. My two favorite statistics about the apps are below.
In the US the market share (in terms of handsets) is currently at 2.4%... When you are developing apps with the intention of using them as advertising vehicles 2.4% is not a very large target to be aiming for. While I acknowledged that this share will rise, Apple will never own the entire market - you will always be aiming for a portion - rather than all - of phone users.Now, once you have reached your 2.4% of the market, or whatever % it may be, you must now contend with the fact that the chances are that people will not use it more than once. The numbers above tell the story quite well.
With poor reach, poor usage, and questionable value-for-advertising-dollars I am not very positive about the marketing potential of iPhone apps.
(By the way I do acknowledge there are some useful fun apps out there, some, and I am talking about this from a marketing perspective)