An article from Wired tells of a court case where a man convicted of murdering his daugter would like to go free because the police used a GPS device to find the grave of the slain girl.
The crux of the case (being taken up by the ACLU, of course) is that the GPS device was installed under a warrant where the cause presented was insufficient.
I was personally pleased to see that they at least went to the trouble of getting a warrant, which I would imagine would not be the case in some jurisdictions. In fact, the prosecutors contend that it really wasn't necessary in this case either, since you can tail somebody on public roadways without a warrant.
In the end, though, this case is centering around whether the police had sufficient reason to watch this guy or not.