Lil' Big Brother?

Some British businesses have taken advantage of a cell phone monitoring service that let's them keep an eye on employees. The idea being to be sure employee's are really "stuck in traffic" or "trapped in bizarre zoo animal hostage situation" when they give an excuse for not being at work.

Also, employers can use the service to make sure employees are safe. So in the event of an emergency, the monitoring employers can notify the proper authorities. Another use is tracking packages and other deliveries for freight services.

Of course, many aren't happy about being monitored. It can be considered a violation of privacy, although employees have to agree to being monitored (this isn't completely clear in the article, however).

So what do you think of this monitoring service? Is it an invasion of privacy, or a legitimate security measure? Could this improve employee efficiency by letting them now they're being monitored? Could using this service hurt businesses by creating a feeling of distrust?