You've asked a really wonderful question, a question that illustrates the IGNORANCE of other drivers who are certain they know the rules and regulations by which they drive, but really don't.
I have been a driving instructor here in California for over 25 years and I am also amazed that many driving instructors don't even know the answer to this question and other similar questions about driving laws.
Your question will undoubtedly educate several other drivers, at least about this rule regarding U-Turns.
Thank you again for asking this question.
YES, IT IS LEGAL TO MAKE A U-TURN OVER A SOLID DOUBLE YELLOW LINE.
You can find your answer in the California Driver's Handbook (http://www.dmv.ca.gov/pubs/hdbk/pgs25thru29.htm )which clearly outlines Legal U-Turns. The following is copied and pasted directly out of the CA DMV Handbook.
Legal U-Turns
A U-turn is turning around in the street to go back the way you came. To make a U-turn, signal and use the far left lane or the center left turn lane. You may make a U-turn:
Across a double yellow line when it is safe and legal.
In a residential district:
if no vehicle approaching you is closer than 200 feet.
whenever a traffic sign or signal protects you from approaching vehicles.
At an intersection on a green light or green arrow unless a "No U-turn" sign is posted.
On a divided highway only if an opening is provided in the center divider.
I hope this answers your question.