An interesting post, from Nick Jones a member of the Gartner blog, where he believes that the ROI of iPhone apps is not obvious given huge numbers of apps available and the small market shares (compare to total smartphones market).
I think this is something to keep in mind when launching an iPhone application, but Nick Jones' calculations doesn't count the value of the iPhone brand which brings a value to an iPhone apps launch. He did mentioned the need to be less emotional and more arithmetic in the pros and cons to use the iPhone, but emotions in brands count, count a lot in fact.
So while arithmetics are important, don’t forget about the marketing aspect of launching an iPhone application. It could generate already way enough return on the buzz talk around the fact that you have an iPhone available, doesn’t really matter if a lot of your customers use it or not… Furthermore, it will let developers (and the marketers too) start to learn distributing their apps on smartphones, even if the technology will change, they can learn the best practices.
Nevertheless, read Nick Jones post, his point of view is worth keeping in mind too before investing in iPhone apps!