That doesn't mean that a warrant is required, nor does it even mean that probable cause is required. But it's clear from the opinion that four, and perhaps five of the justices, would hold that long-term monitoring of a vehicle (how long would probably be dependent on the facts of the particular case) violates a reasonable expectation of privacy, and likely would require a warrant.
On the other hand, short-term monitoring of the GPS device, while it is a search, might not require a warrant. In fact, the warrant requirement for a short term monitoring revolves around whether the government has to get a warrant to install the device in the first place. If the installation requires a warrant, then the government's right to monitor the device without a warrant becomes moot.
What does the Court's jumbled opinion mean for employers that engage in GPS or other remote monitoring of their vehicles? Where monitoring is a general aspect of the job, e.g., with delivery vehicles that are GPS equipped, or workers who travel with company provided, and the GPS enabled, smartphones, things shouldn't change. There is no reasonable expectation of privacy (which is the initial standard by which most state laws assess whether an employer goes too far in watching its workforce) in a situation where an employee using a company provided vehicle or phone is aware of the GPS tracking. But in situations where employers surreptitiously use tracking devices, the decision hints at a red flag.
For example, some employers use private investigators to surreptitiously observe employees who are out on workers compensation leave, to see if they are actually as injured as they claim. These observations frequently involve the placement of a tracking device on an absent employee's vehicle so that the investigator doesn't have to maintain 24-hour coverage to see the employee doing something inconsistent with his workers compensation injury status. Depending on the circumstances, both the placement and the monitoring of such device would likely intrude on employee's reasonable expectation of privacy, as delineated in Jones.
Moreover, the decision has been widely misreported in the press, and raises the awareness and expectation of employees with respect to their right to privacy. Under these circumstances, an employer should look carefully at state law before it or its agents decide to make use of GPS assisted oversight without an employee's knowledge.
UPDATE: Here's a brief discussion from LawBlog on the application to private citizens.