In Snow Leopard, hold down the Option key before clicking the AirPort menu. Doing so reveals additional technical details including which standards, speeds, and frequencies you're using to connect, as well as what's in use by other networks. With the Option key held down and with a network already joined, the AirPort menu reveals seven pieces of information: the PHY Mode, the MAC (Media Access Control) address, the channel and band in use, the security method that's in use, the RSSI (Received Signal Strength Indication) measurement, the transmit rate, and the MCS Index. In Leopard, some, but not all, of these details are revealed by Option-clicking the AirPort menu.

From Doug McLean