Do yourself a favor and check out Power Hero, which benjie linked above. You can turn off several functions (including the radio) after a configurable amount of idle time. This saves me a lot of battery life here in upstate NY, where there are several dead spots between home and work, and my phone would constantly be polling for the home network.
The only feature I haven't been able to use is the auto-off for Bluetooth. When I tried, all of my calls would go to voicemail by the time Power Hero renabled BT and the headset reconnected.