QuickerTek has introduced the Dualy Extreme, a transmitter that upgrades the Apple AirPort Extreme Base Station (802.11n) with additional power. It's priced starting at $350.
The Dualy Extreme provides 1 watt of RF power, thus enabling the base station to transmit up to five times farther than its standard wireless range. The added power is coupled to two antennas.
The upgrade can be installed on the base station for you, if you already own one; alternately, QuickerTek offers upgraded base stations as well (the upgrade kit costs $350, a full base station is priced at $580).
QuickerTek offers the kit either with two 500 milliwatt transceivers or two one-watt transceivers for an additional fee, for even greater range.