Here's a frightening but real proposition: if you are caught breaking certain traffic laws, not only do police have the right to search you—they can go through all your electronic data as well—your text messages, call histories, browsing history, downloaded emails and photos. In a recent academic paper, South Texas Assistant Professor Adam Gershowitz explains that because many traffic violations are arrestable offences, just as a cop could search your pockets for drugs, said cop can also search your pockets for a smartphone and go through all its contents. The same is true for any standard arrest, and given the amount of data in current smartphones, it's a scary proposition (even for law-abiding citizens like us).
We'll give you the CliffsNotes version of Gershowitz's 30-page article in which he outlines the situation.
While society and technology have changed drastically over the last few decades, the search incident to arrest rule has remained static. Thus, if we think of an iPhone as a container like a cigarette package or a closed box, police can open and search the contents inside with no questions asked and no probable cause required, so long as they are doing so pursuant to a valid arrest.
A Recent Precedent:
The Fifth Circuit's recent 2007 in United States v. Finley is representative. Police arrested Finley after a staged drug sale. The police then searched Finley incident to arrest and found a cellphone in his pocket. One of the investigating officers searched through the phone's records and found text messages that appeared to relate to drug trafficking...the court explained that "police officers are not constrained to search only for weapons...they may also, without any additional justification, look for evidence of the arrestee's crime on his person in order to preserve it for use at trial.
Courts and legislatures can attempt to minimize this invasion of privacy by changing the legal rules to require that searches be related to the purpose of the arrest, by limiting searches to applications that are already open, by restricting suspicionless investigation to a small number of discrete steps, or by limiting searches to data already downloaded onto the iPhone, rather than data that is merely accessible through the iPhone's internet connection.
AU: Obviously Australian laws are different to the US, but this is still frighteningly relevant to remember.