In a court ruling that came out a little while ago (just catching up now), Judge Richard Posner took the lead in an appeals court ruling that effectively reaffirmed the idea that police don't need a warrant to search mobile phones
as they're arresting someone. Of course, this general concept is not new
and I've discussed my concerns about police being able to search phones without a warrant in the past -- but this particular ruling does seem pretty limited. While Posner notes some of the bigger questions, he basically compares the phone to a diary, and focuses on the mere searching of basic data, like the address book, to suggest this particular search was limited, and doesn't raise any significant 4th amendment issue.
It’s not even clear that we need a rule of law specific
to cell phones or other computers. If police are entitled
to open a pocket diary to copy the owner’s address, they
should be entitled to turn on a cell phone to learn its
number. If allowed to leaf through a pocket address
book, as they are, United States v. Rodriguez, 995 F.2d 776,
778 (7th Cir. 1993), they should be entitled to read the
address book in a cell phone. If forbidden to peruse love
letters recognized as such found wedged between the
pages of the address book, they should be forbidden to
read love letters in the files of a cell phone.
Effectively, the court more or less says that circumstances matter, and that in situations where there's a chance that data will get deleted or modified, it's perfectly reasonable to do a simple
search of the mobile phone, but it could get thornier if it moves more in the direction of voyeurism, rather than as evidence for crimes. I'd certainly prefer much stronger privacy protections -- especially as a phone really is a window into much, much more than just a phone, but the ruling isn't much of a surprise and seems to align with other rulings on similar issues.