Low end smartphones will need less power and (if the battery is sized right) last longer than what they're now dubbing 'superphones' (high end smartphones)
If you're looking for processing power when you want it, but power sipping when idle, look for an android device and one that has lots of community support.
You can root it and remove everything you'll never use, disable any and all background processes that aren't necessary, etc.
If you got a nexus device you can disable pretty much everything without needing root, as there's no bloatware installed.
If I turn off mobile data, sync services, and all background programs, my nexus one would probably last a week if I never made calls/texts. The mobile data and screen are the two biggest draws for me. Cell connection is only a factor in low signal areas, when the device pumps up the power to stay connected. If you have 4+ bars it shouldn't take much power.