There is a good overview of the ongoing court case United States of America v. National Association of Realtors found in the article Regulatory Body Grappling With Online Real Estate Businesses [E-Commerce Times] 
In the 1990’s, real estate brokers began creating password protected web sites that allowed customers to search private databases called Virtual Office Web Sites (VOWs). They replace the broker in the searching of potential listings, allowing the customers to educate themselves before contacting a broker.
The recent NAR policy of requiring a broker to opt out is they present listings on the internet without the permission of the listing broker.
“The DOJ presents a convincing argument in favor of technological innovation. With the increase in the use of the Internet in business transactions, the time has come for real estate licensing and regulatory bodies to embrace technology. Rather than fighting the inevitable, they should incorporate technology into industry so as to conserve resources and encourage competition while at the same time, adopting measures to safeguard the public.
Whatever the outcome of this case, it is sure to have broad implications on e-commerce and the ability of industry regulatory bodies to maintain their traditional grip on their industry when their members are doing business online.”