The first killer-app for mobiles is the phone calls. I believe static downloadable audio contents such as music, audio books, and podcasts will be the next killer-app. Only design problem is sharing one battery for both critical (phone calls) and casual (listening to audio) uses which calls for smarter power management and more powerful batteries.
Great user experience and clever hardware design is where Apple excels at so I expect Apple to introduce iPod phones within a year or two at the latest.
Unfortunately, screen-based applications and dynamic audio contents, driven by voice browser technologies, will continue to struggle in the near future due to tedious user experience. Mobile killer apps must be available at the user's fingertip, not hidden behind a wall of patience.