Google recently came out with a new service called Latitude which allows people to share their locations with each other via a Google Maps interface.  Almost immediately, talk about privacy concerns dominated the dialogue concerning Latitude. These fears, upon closer scrutiny, are largely baseless. Latitude does not present a significant danger to users' privacy; any suggestion otherwise is mere technophobia and headline-grabbing Google-bashing.
The most important point in this entire conversation is that your cell phone is already a tracking device in and of itself. Carrying around a cell phone surreptitiously exposes more personal information than Latitude could ever dream of doing. The GPS, wireless Internet, and cell phone signals that emanate from your phone can be used to locate you any time your phone is on. The cell phone companies, obviously, know your location because they need it to deliver you service; the government can get it via Triggerfish or by just asking the phone companies. But a phone can be used as more than just a locator -- it can also be used as an eavesdropper. Consider the well-known NSA surveillance program that slurps up cell phone conversations, or the ability of the government to listen to whatever noise a cell phone picks up even when it is powered off. If you are seriously worried about your privacy, you won't even be carrying around a phone in the first place.
Google Latitude can not honestly be called a privacy threat because it is opt-in at every level and gives one the opportunity to leave or disable the service at any time. For another person to have access to your location, you must 1) explicitly enable Latitude 2) request the other person to receive your location via Latitude or accept a similar request from him 3) not turn the service off. Disabling the service can come in the form of either opting out of Latitude entirely or hiding your location temporarily. You can even enable a 'city-level-only' location option, which only shares your location to the town level of granularity, or set a manuallocation that doesn't move. (Your mobile location can be exactly determined only if you install Latitude on your mobile phone as opposed to using the stationary option.) Again, no one besides the group of people you explicitly agree to share your location with can see your location.
The example scenarios that have been raised by Privacy International with regard to Latitude's purported privacy degradation that have captured headlines are pretty far-fetched. All of the scenarios involve a malicious user creating a Google account, enabling Latitude on a phone and giving the phone to someone else with the intention of tracking them (without, of course, informing them that Latitude is enabled on the phone). Any reasonably competent person would quickly discover that Latitude was enabled on the phone, if he had not inspected the phone in the first place when he initially received it. There are many other major invasions of privacy taking place elsewhere, and Privacy International would do well to raise a stink about those issues rather than chase windmills at the Googleplex.
There is a legitimate privacy concern that Google will store the history of a user's location, which could be used to construct a profile of where a user was at certain points in time. However, Google states in the Latitude FAQ that this is not the case: "Google Latitude only reports your last updated location and does not keep a history of previously reported locations." As long as Google keeps its word in this regard, and I believe that to be a reasonably safe assumption, there is no privacy danger here.
It is unfortunate that so much ado has been made about a service that is essentially a useful visualization of your friend group.  Google Latitude is a service that you should have no qualms about using, provided that carrying around a cell phone does not make you queasy.
 As several other commentators on Slashdot pointed out, Google is not the first company to offer this kind of service (Brightkite, Loopt, and Mologogo to name just a few).
 One could imagine other use cases: giving truckers cell phones to track their shipments, planning visits to friends based on their proximity to a certain destination, serendipitousmeetup opportunities with nearby friends, etc.