FBI can put GPS in your car sans warrant
Student Who Found GPS Device On His Car Due To Reddit Comment Sues The FBI (2011-03-04) By Mike Masnick. @ Source www.techdirt.com
Here's a comment by the well know computer security expert Bruce Schneier.
The FBI is Tracking Whom? (2010-10-13) By Bruce Schneier @ Source www.schneier.com
Time mag has a article about a related issue:
The Government Can Use GPS to Track Your Moves (2010-08-25) By Adam Cohen. @ Source www.time.com