I created a mod so I can turn off the cellular radio, but keep WiFi and Bluetooth on. There is no current way to do this, only the "Airplane Mode" which shuts off ALL radios!

I currently do not have Sprint service on my Pre, and am just using it with WiFi. This would also be useful for people wanting to save their battery if in a poor or no service area and wish to still use wifi. Another use is shutting off the phone so calls will not be received.

I posted the howto in the wiki:
pre dev wiki: Radio Power Switch