Rafe's comments
Every developer should take a look at Nokia's Energy Profiler application. Why? Because energy usage (or battery life) is already a major constraint on mobile devices and it is only going to get more important. While memory (both RAM and disk space) is increasing along with processing power, a commensurate improvement in battery technology is not taking place. This means power management is going to become the most important constraint when developing mobile applications.
Developers can use Energy Profile to see how their application performs. My personal recommendation would to pay the most attention to the time when your application should be in a dormant state.