For fellow Californians only,
The Office of the Attorney General has set up a page where you can pre-registerfor the nationwide telemarketing Do Not Call Registry. You can use this form to register up to three phone numbers. Enforcement of the “no call” ban is scheduled to begin in October 2003.
Updated 8/21/2012:Fixed some links and info. Since I posted this, the law took effect and was strengthened in 2008.