If a motorist calls 911 and reports having been run off the road, is that sufficient cause for police to stop a vehicle fitting the caller's description?  I would think so.  The Supreme Court decided it was this morning, in a surprisingly close 5-4 decision, Navarette v. California.

For those who like to keep track of ideological designations, the split indicates once again that a one-dimensional liberal-conservative model doesn't hack it.  Libertarianism is an different (though not necessarily orthogonal) dimension.  Justice Breyer was in the majority favoring the government; Justice Scalia wrote the dissent.