They can transmit and receive all by their lonesome. All that is necessary to do what you want (have two cell phones talk to each other without a base station, cell tower, etc.) is the appropriate software. Carriers often lock things down (e.g. try to prohibit VOIP), so usually it's not easy, but the hardware is capable of doing what you want.

At least as I understand it. :-)

OpenGarden may do what you want right now. Dunno.

Cheers,
Scott.