I'm going out on a limb here and say that this whole GPS thing may be constitutional. But it probably should be prohibited (i.e., that states should pass laws limiting the practice).
No, you don't have a reasonable expectation of privacy while you're out in the street. Would you be okay if someone assigned a police officer to shadow you?
I don't doubt it will somehow be ruled Constitutional. What remains to be seen is what happens if the person finds the GPS thingie. Is it legal for them to remove it? Do they legally have to return it, at their own expense, to the original owner? Can ordinary folks bug vehicles as well, or only police? What's the law stating so? Back in the day, I was trained to look under my HMMWV for any leaks or IED's. I find it hilarious that now I should periodically check to see if I'm being watched by (hopefully) the police. For all I know, it could be fistful that's bugging my car.
As an infosec buff, I can't but see the potential for entertainment. Scan your car with an RF field strength meter (HAM folks sell them) and visually, find an tracking device, use gloves to remove, wrap it in RF damping material, attach it to the car of a mayor/DA/lawyer/civil rights person/journalist and then call up the local papers "There may be a police GPS tracking device attached to XYZ's car, might want to ask him about it" from a throw away prepaid cell. I'd never do so, seriously, but I imagine it's only a matter of time until it happens. Or the police beat us all to the punch by doing so first.
Personally, if I could obviously tell it was not an IED, I would remove it and leave it on my porch with a note taped to it. If I couldn't absolutely tell if it was not an IED, I'd call the police, explain that I have a strange foreign device strapped to my car that I can't identify and request the bomb squad be sent out.
