Mark made this wonderful demo showing how you can use port 4 (RS485) to communicate between multiple (three, in this demonstration) NXTs. The video explains well how this is done, and show how to connect two NXTs (via a regular cable), 4 NXTs via mindsensors.com port splitter, and even wiring NXTs 100 feet apart!
The NXTs were programmed using NXC, and the source codes can be downloaded here. The codes are modified versions of John Hansen sample RS485 files.

UPDATE: Philo sent me the following comment:

It is a common but very bad practice to use only two wires to connect RS485 devices: the ground should be conneced too, even though RS485 uses differential lines. Though it can work, as exemplified in the video, ommitting the ground may cause subtle failures and even dramatic ones (destroyed RS485 driver).

The drivers used in the NXT withstand a ground potential difference of only -7V to +12V. This might be exceeded if the connected NXTs have other external connection, eg to the mains via the battery/adapter.