iPhone supports Web 2.0

Though Apple is not allowing 3rd party developers (yet) to install applications directly on the iPhone, they are fully supporting Web 2.0 so application developers can write server based systems.

This will mean there is interaction over air, but with US carrier pricing adopting unlimited data plans, this will allow highly interactive applications.

If the iPhone really is running MacOS X and the full set of application libraries it should allow developers to relatively easily port Mac application over to it, if Apple then allow them to be stored on the phone itself.