Is there an app to disable everything but GPS on iPhone? I use my iPhone 4 as a GPS device when I go tramping, but since the phone is constantly searching for signal in remote locations it drains through the battery extremely quickly. 

The simple solution is to remove the sim card, although I've lost one microsim this way, and it's not an ideal solution.

I'm looking for a method to disable the phone/transmitting functions, while still being able to use GPS. Does anyone know of any apps (I guess from the Cydia store - I doubt Apple would allow this) or method to achieve this?

Edit: Just to be clear, what I'm meaning is that all transmitting functions on the iPhone are turned off - the only thing that should still be functioning is the GPS.

The 'phone' toggle in SBSettings doesn't work because it disables most things including GPS, and just leaves wifi enabled.

Update: Just to be clear, the same result I'm looking for can be achieved by removing the microsim, it's just that it feels like there should be an easier way to accomplish this without removing the sim.
If jailbroken, SBSettings allows one to turn off everything: Data, Wifi, Bluetooth, 3G, Edge, Phone, SSH, etc. but leave GPS enabled.

If not jailbroken, Settings > General > Network lets you turn off WiFi, 3G, BlueTooth, and Data, which leaves GPS, Edge, and Phone. Turn brightness all the way down. This ought to extend battery life quite a bit.