The three fundamental "juices" of mobile devices - storage, CPU, and bandwidth - have been improving over the past decade. Storage has grown thanks to better and cheaper flash memory, CPUs have become spiffy thanks to Moore's law and advances in low-power CPU design, and bandwidth has improved thanks to 3G network roll outs. These 3 define the operating point of mobile device applications i.e., what are the trade-offs application designers make based on these underlying capabilities of a device. For example, if bandwidth was free and infinite then there would be less motivation to cache data on devices and instead most data (e.g. an address book) would be stored online. This would have the advantage of letting users access the same address book from different devices and computers.
But the truth is that while storage (bits per dollar) has doubled almost every year in the last decade, CPU and bandwidth have not kept the same pace. CPU power can still be jacked up if one is willing to trade battery life for faster CPUs. But bandwidth remains expensive (bits per dollar) and unreliable (spotty coverage) as compared to storage. Where is the operating point of upcoming mobile devices like smartphones heading? Will the distortion due to seemingly limitless storage lead to a redefinition of the mobile phone into a device that is less phone and more storage/CPU? Will voice and data lose their killer application status on the mobile device? What will users be doing with all that storage and CPU becoming available on their mobile phones?
It is safe to state that whenever data can be cached it will be cached. It just costs much more to transmit a bit than to store it, and transmission latency is also a problem. So expect applications to reduce bandwidth usage through local caching. High cellular bandwidth cost will also promote disruptive services like Wifi-enabled smartphones that can easily bypass operators' expensive data networks. With applications like Fring, users can already make phone calls using Skype on Wifi enabled phones. Mobile phone cameras routinely take images at multiple megapixel resolutions. But these high resolution pictures are seldom transmitted over the mobile wireless interface and are instead stored for later downloading to a PC. Many mobile device mapping solutions do not use cellular data networks and instead rely on storing all the maps locally. Multiplayer mobile games have not taken off either, partly because of the technical and pricing limitations of cellular bandwidth. I download my email over my home Wifi connection into my Nokia E71, reply to these emails on the train while commuting to work, and then connect via Wifi to send the replies once I am in office. Similarly, applications like Avantgo give users the option of avoiding the usage of expensive cellular data plans.
The argument I have tried to make is that the mobile device is not shackled to voice and cellular data only. Market forces will spur creativity to utilize these little wonders of modern technology in engaging and inexpensive ways. Voice and data may not remain the killer-apps on these devices.