The St. Petersburg Times just posted an article entitled "Should authorities need a warrant to put a GPS tracking device on your car?"
The article doesn't break new ground legally, but does contain an interesting discussion of how law enforcement agencies work around different state and federal laws, rules, and standards:
How GPS tracking is used in Florida depends on which agency is using it. Local, state and federal officers follow different rules. According to Florida law, local and state agencies need a judge's approval to use GPS tracking. But the standard they have to meet isn't as high as the standard for obtaining a search warrant. To search someone's home or business, officers must have a reasonable belief that the person committed a crime. To use GPS tracking, they simply must convince a judge that it's "relevant" to their investigation, said University of Florida law professor Michael L. Seigel. "It's a much lower standard," he said. "It's not requiring them to show any suspicion about an individual's guilt." There's also an easy way around state law. Local agencies could just ask the federal government for help. Federal agents don't need a warrant to use GPS tracking devices in Florida, Seigel said. "Everybody's working in a joint task force these days," said Escobar. "They can ask the federal government to do things they can't do in the state system."
I have written a bit on this topic See here on the blog and here for a draft of an article I am publishing on the topic this winter. As I noted before, I think the warrantless use of GPS tracking devices by law enforcement presents significant Fourth Amendment issues.