The FCC has a new and somewhat baroque regulation that legalizes plugging aftermarket antennae into your WiFi access-points. In a long post on WiFiNetNews, Glenn Fleishman takes apart the new regulation and explains in admirably.
The FCC rule doesn't suddenly make all antennas legal for all systems. Instead, they have chosen a clever middle ground. For new devices—or, presumably for recertification of old devices—manufacturers will be allowed to test the system with high-gain antennas of each major type, like omni, patch, yagi, and so forth. Once the device is certified, the manufacturer can release the characteristics of the antennas they tested for both their in-band and out-of-band signal patterns and strengths. (Out-of-band transmissions are the inevitable but not intentional frequencies that are broadcast on at typically very low levels due to harmonics and other technical radio issues.)
(Thanks, Glenn!)