There has recently been some controversy in the United States about the lawfulness of police "searching" (ie reading the contents of) cellphones and smartphones of individuals who have been arrested, and whether it is just an ordinary search "incident to arrest". (See, for example, Police push for warrantless searches of cell phones | Politics and Law - CNET News.)
At least in Canada, the Ontario Superior Court of Justice has said it's a no-no.
Following an arrest in connection with a heroin importation investigation, police seized a cellular phone from one of the suspects and subsequently perused the contents. The phone revealed a number of text messages, which were believed to be to and from an alleged co-conspirator. Police later exchanged text messages with the suspect and phoned the number after the suspect had been arrested to confirm that he was in possession of the relevant phone. The defendant argued that the search of the cell phone was an unlawful breach of the defendant’s privacy rights and should be excluded under the Charter. The Court determined, in R. v. Burchell, 2011 ONSC 6236 (available on Quicklaw, CanLii link to come), that the evidence should be excluded as reviewing the contents of the phone was unlawful.
The Court said, at paragraph 55:
“If the police had found a sealed box of files in the applicant's vehicle, no one would credibly argue that the officers could conduct a detailed analysis of the files as part of their power to search incident to arrest. There is no reason in principle why the search of a phone should be treated any differently.”