Back in the day, the white dome Airport Base Station supported Power over Ethernet, which allowed the station to be mounted far away from an outlet (e.g. on a ceiling) powered by just the ethernet data cable, great for preexisting buildings where running power for base stations might be expensive or impossible.
The most recent Airport Extreme Base Stations have lacked this convenient feature, but MacWireless is now offering a PoE kit that works with the 802.11n AEBS. The $59.98 kit contains both an injector that plugs into the outlet and a splitter for the other end that goes to the AEBS, allowing both data and power to be sent over a single Cat5 cable up to 330 ft long. A shorter 200 ft range kit is $39.98.