You would think that police would require a warrant to get your phone's location info, right? Not according to the US' Fourth Circuit Court of Appeals. It just ruled that asking a company for cellphone location data you've offered to a third-party doesn't represent a search under the Constitution's Fourth Amendment, and thus doesn't need a warrant. According to the judges voting for the decision, volunteering your position info means you've given up a "reasonable expectation of privacy" -- if you didn't want to share where you were, you wouldn't have handed that knowledge over to someone else, would you?
The ruling is in line with what some other courts have said, and ends a split between courts on the topic. However, it won't be surprising if there's an eventual Supreme Court challenge. As many would note, it's virtually impossible to avoid supplying your location at some point. Making a call will offer some basic positional info to your carrier, and many common smartphone tasks (such as navigation or social check-ins) demand that data. Until there's a ruling to the contrary, though, you can't assume that the police will have to jump through hoops to find out where you've been.