On Tuesday prosecutors for the Obama administration argued that records of location data gathered by cell-phone companies should be available to law enforcement even when no search warrant has previously been issued by a judge.
In other words, If Uncle Sam wins on this argument, every law-enforcement agency in the country will be able to track your every move. More importantly, access to location data as comprehensive as that available to cell-phone carriers could allow law enforcement to determine everything from your complete social network and your your health status to how likely it is that you'll repay a loan.
The case at hand does not suggest that the Obama administration is attempting to gain this level of insight into the lives of every American citizen, but it's telling that the prosecutors seem ignorant of the power of the data they're requesting.
To understand how important location data is, especially of the variety gathered by smartphones, it's important to understand what academics have already accomplished with this data.