The biggest problem with deploying an HTML webapp to iPhone users is that they aren't trained to accept them. Users are forced to learn to use the App Store, but virtually none of them know how to make a bookmark on the home screens and even fewer know that this can then enable a website to become an application. (For those not washed in the gory details, full screen is not available until the user relaunches from a home screen icon.)

All together, you will need to have a motivated and technically adept audience for your webapp to succeed.