It's now clear that police don't need a warrant to track your cellphone, but searching that phone is another matter; there's no obvious guiding policy. Any murkiness may be settled soon, as the Supreme Court has agreed to rule on two cases where the accused have objected to cops obtaining evidence from their phones without warrants. Decisions in either case could set precedents for searches across the US; if judges determine that warrants are necessary, they could challenge guidelines in California and other places that allow warrantless searches after arrests. The court hasn't scheduled the relevant hearings, though, so it may be a while before there are any definitive answers.
[Image credit: Erin Nekervis, Flickr]