Radio emissions in the U.S. are refereed by the FCC. In Title 47, Chapter I, Part 15, Subpart C, Section 15.247, rules pertaining to unlicensed operations in the 2.4GHz range are outlined.
The full text of Part 15 (and most other sections of US federal code) can be browsed online at http://www.access.gpo.gov/nara/cfr/cfr-table-search.html.
For a good discussion (and interesting interpretation) of the rules and how they pertain specifically to 802.11b, take a look at Tim Pozar’s excellent report for the BAWUG, available online at http://www.lns.com/papers/FCCPart15_and_the_ISM_2.4G_Band.index.
From the U.S. Government Printing Office via GPO Access
CHAPTER I—FEDERAL COMMUNICATIONS COMMISSION
PART 15—RADIO FREQUENCY DEVICES—Table of Contents
Subpart C—Intentional Radiators
Sec. 15.247 Operation within the bands 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz.
(a) Operation under the provisions of this section is limited to frequency hopping and direct sequence spread spectrum intentional radiators that comply with the following provisions:
(1) Frequency hopping systems shall have hopping channel carrier frequencies separated by a minimum of 25 kHz or the 20 dB bandwidth of the hopping channel, whichever is greater. The system shall hop to channel frequencies that are selected at the system hopping rate from a pseudorandomly ordered list of hopping frequencies. Each frequency must be used equally on the average by each transmitter. ...