Trends: Cell tracking getting more coverage
The Guardian has an article about how they set up a phone to be tracked using an online service, and were able to track someone for a week, knowing at all time where she was. This is not even a GPS phone, a simple GSM phone can be tracked using sometimes free services.
The article doesn't say which service was used, but chances are it's world-tracker.com. These services usually request a confirmation from the phone itself so abuse is limited, unless you have access to that cell phone in that person's back. But it goes to show how easy tracking is with cell networks. In the US, it's a big controversy right now as to if law enforcement should be able to track people with their cell phones without a warrant, but soon anyone will be able to do so on the Internet, sorta like how search engines like Google allow people to find anything that is public about anyone.