Interesting – I didn’t know that such services were generally available yet…
For a cost of about 35-50p, Trace A Mobile will tell you the location of a mobile phone. You need to sign up for a monthly subscription, and you need to have the advance approval of the phone user (usually established by sending them a text message) but for nervous parents, or for spouses who need to know how close their husband is to home, this could be a good service.
Of course, it would generally be cheaper just to call them and ask. So this may be better used for locating your lost Irish Wolfhound, or your stolen Ferarri.
A friend of mine was recently beat up and robbed. His iphone was stolen. He got it back pretty quick though when the thieves tried to use it after he reported it stolen. Apparently ATT uses this feature to track theft. I wish i had more information, i heard all of this through the grapevine.