Some hacker is probably going to use this to stalk people
Sadly I could see that being the case. Worse part is they will not use some security hole or something Apple could patch to stop it from happening. People are just way to free with giving away that infomation publicly and then they get freaked out that people can follow them every where.
I have Google Latitude and on that same token 99% of the time I have it turned off and I do not have any info sent to it. The few times I do use it I am very very selective who even has access to the info. Last time I used it was over a year ago and that was when I was going to visit a girl I was dating so she could keep track of were I was on the road so she could have a good ETA and knew when I was getting close but I leave it turned off.
For me a service like this would never really ever be turned on and I have a feeling that it will be a huge failure in the end since it will be linked only to Apple devices.