I'd argue that just because it hasn't been ruled unconstitutional doesn't make it constitutional.
Look, I've got nothing to hide, but please explain to me where the government gets the authority to track my movements? Think about it this way, this technology is still relatively in it's infancy, but imagine 5-10 years from now when prices have come down to the point that entire fleets have this technology on them. It will be virtually impossible for somebody to conduct their life without the government having a record of where they were, and when they were there. So what happens when I go to the grocery store and the bank next door gets robbed, the local PD goes into their database, takes a list of all the plates that were in the area and then questions all those people? Why should I have to explain why I was in a public area to an official from the government? That doesn't even begin to delve into the possibilities for abuse of the system, say the jealous husband who wants to see where his wife is while he's at work, or the town Mayor (who exerts a TON of control over the PD, especially in a smaller town) who wants to know who his political opponent is talking to and where.
Like I said, I do understand the investigational benefits of a system like this, I just feel that the privacy concerns outweigh the benefits. Again, a read & dump system I would have much less of a problem with, it's the database that scares me.