This is one of those things that makes me a bit leery of carrying a mobile. This is in Washington State, but apparently the California Supreme Court ruled in a similar manner.
Mobile phones exist in a constitutional grey area. The law has well-developed doctrines protecting the privacy of our desktop computers, landline telephones, and filing cabinets. But modern cell phones perform all of these functions, and more. If the police are free to rummage through any cell phone that falls into their hands, every arrest would automatically give the police access to a treasure trove of private data that they would otherwise need a warrant, based on probable cause, to obtain.