Unitron's and nomamtter_nevermind's comments make me realize how little I actually know about the detailed rules for when a warrant is required and when it's not. I think there should be a difference between when a cell phone can be seized as an object and when the information on the cell phone can be accessed. I know, though, that the California supreme court held awhile ago that police can search the entire contents of a cell phone without a warrant, incident to an arrest. (I think that ruling would very possibly be overturned if it gets to the U.S. Supreme Court.)