Giant Ant has published a paper describing ten heuristics for mobile design. A nice, clear encapsulation of some of the primary issues for designers of mobile devices.
“5. The device as continuous companion opens the realm for mobile experiences of different intensities and durations. With the device always present, there is opportunity for multiple interactions within a day or week. In addition to engagement over time, games and fast tasks can fit within users’ everyday activities. Asynchronic communication, including the popularity of text messaging, points to new interaction models that do not interrupt other activities.”
Link: Design Sketch: The Context of Mobile Interaction (giantant.com, 180k PDF)