If you read the actual opinion (which I recommend, it's very interesting), placing the tracking device on the vehicle's undercarriage is actually trespass:

"It is important to be clear about what occurred in this case: The Government physically occupied private property for the purpose of obtaining information. We have no doubt that such a physical intrusion would have been considered a “search” within the meaning of the Fourth Amendment when it was adopted." (page 4 of the opinion, 6 of the pdf)

http://www.supremecourt.gov/opinions/11pdf/10-1259.pdf