In the United States, the Federal Communications Commission (FCC) regulates the use of antennas with Wi-Fi as part of its general regulation of radio frequency devices. You can find the full text of the (rather long and detailed) regulations, which make up Title 47 of Part 15 of the Code of Federal Regulations (CFR) at http://www.access.gpo.gov/nara/cfr/waisidx_02/47cfr15_02.html. So that you don't actually have to read this lengthy bureaucratic document, I'll summarize the parts of here it that might matter to you.
These rules create a power limitation for Wi-Fi networks. From the viewpoint of the FCC regulations, the power of a Wi-Fi broadcast is measured in units of equivalent isotropically radiated power (EIRP). EIRP represents the total effective transmitting power of the radio in a Wi-Fi card or access point, including adding gains from an antenna and subtracting losses from an antenna cable. When using an omnidirectional antenna (see "Different Kinds of Antennas" earlier in this chapter for more information about types of antennas) with fewer than 6 decibels (dB) gain, the FCC requires EIRP to be under one watt.