As a poof-of-concept, how about a bar-code reading app for the iPhone? Users could then either consult a previously-downloaded db or else connect to one on a server somewhere.

I think this could be huge, leading to everything on the consumer side from self-generating shopping lists/wish lists to micro-purchases that either debit a paypal account or go on your phone bill. On the producer side, I see iPhones as enterprise tools for managing inventory and doubtless many other uses.