http://thelede.blogs.nytimes.com/2011/05/18/x-ray-scan-reveals-513-mexican-migrants-in-2-trucks/

I can see how this would be very usefull in border crossings to help prevent against illegial immigration and in ATFP(anit-terriorisom force protection) But as the article says that it could and is being used in cop cars driving past vehicles.  So where do we draw the line? if you drive past a car with something in it, then how do we not say that whatever the car was pulled over was frivilious? When does this become a violation of the 4th amendment? when does probable cause go out the window?