According to law enforcement agencies, the California Vehicle Code doesn’t specifically say that lane-splitting is illegal. That doesn’t mean a motorcyclist might not get ticketed for it, though.
Motorcyclists could still be cited for unsafe driving and it’s up to the discretion of the ticketing officer and the court to determine what’s safe.
The California Highway Patrol’s Web site says lane splitting “is permissible but must be done in a safe and prudent manner,” while the DMV’s Motorcycle Handbook warns that lane sharing is dangerous.
San Diego police Lt. Tim Saelens, who works in the traffic division, said motorcyclists who split lanes are sometimes cited under three sections of the state Vehicle Code: Section 22350, the basic speed law; Section 21658, which governs lane changes; and Section 21755, that allows passing on the right only under safe conditions.
From http://www.signonsandiego.com/news/2010/feb/19/sign-of-trouble/ via a tip from Zina at http://piratemonkeycult.wordpress.com/2010/02/19/molesters-and-whatnots/