It sounds like this is like when taxi drivers have to show off the ID number in their cab. There's a new law coming, reports the Real Estate Blog.
"California Senate Bill No. 1461 is effective as of July 1, 2009, and it has real estate agents scrambling to comply. This Bill amends Section 10140.6 of the Business and Professions Code. As of July 1st, it requires a real estate '... licensee to include his or her license identification number on specified solicitation materials, as defined, and on real property purchase agreements when acting as an agent in those transactions.' "