You should build one if your customers ask for it.
And if you do well they will ask. Most iPhone apps are just web-apps w/a "phone-top" interface. Running your app through Safari on the iPhone feels too slow and looks crappy next to a native app. I think having an iPhone app is going to be as standard as having website soon, if not already.
Also agree w/ sibling poster about access to your contacts + other richness.
Otherwise, good luck! I really like how you ritually slaughtered all the BS.
My primary envisaged use for your app was on the move. I have a phone, and need to talk to several people at once. No problem, conference call. Whip out the app, add a few contacts, have the conference call.