Vehicles on the other hand, even when they are on public property still belong to the owner and hence although no trespassing is required in planting the device (inside or outside the vehicle), the Feds would still be meddling with private property.
"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)
In this case, you could argue the garbage becomes public properly only after it is collected, and only by the collection services, which you would expect on regular basis. Thus you'd prevent FBI from just pick up your trash, saying "well, you were gonna give it up anyway".
Thoughts?
You could make that argument, but I'd disagree. Law enforcement can (should) only do what we the people allow.