First, understand that the search of your cell phone, at a minimum, would have to be after you were arrested for some crime. California allows it because it is a “search incident to arrest.” http://articles.cnn.com/2011-05-31/tech/warrantless.phone.searches_1_cell-phone-police-search-warrant-requirement?_s=PM:TECH Understand that a search incident to arrest has some gray areas in it, for that you can be sure. See, for example, http://law.onecle.com/constitution/amendment-04/14-search-incident-to-arrest.html Generally speaking, such an arrest is to prevent destruction of evidence and to prevent access to a weapon, the latter of which the police are more worried about.
The law is constantly evolving. One thing that I think is predictable is that the United States Supreme Court will review this at some point. Remember back in January 2012 when the Justices held that a GPS device being placed on a vehicle did require the police to obtain a warrant and not just hide it without notice to anyone (including a judge) that it was being done? http://www.usatoday.com/news/washington/judicial/story/2012-01-23/supreme-court-GPS/52754354/1 Thus, as technology advances, the courts will deal more and more with such things as searches of a cell phone without a warrant and whether it can be done for any arrest (public drunk?), just for felonies, or just when the police can articulate a reason for doing so in emergency (what the courts call “exigent”) circumstances. Until then, maybe you need to use a pattern screen lock on an Android smartphone. Seems that the FBI cannot crack that security: http://www.wired.com/threatlevel/2012/03/fbi-android-phone-lock/?