Facebook is rolling out a new feature called "Places", this is very much like the Foursquare and Gowalla check-ins.
This is a neat feature, but, and it's a big BUT, you better check your settings.
The big one is the option to allow others to check you in. This can be very, very bad. As I posted on my FaceBook status, you need to disable this option. This will keep folks from pranking you by checking you in to some, let's say, not so nice places. Such as, adult bookstore, strip club, the drug corner down the street. Obviously you wouldn't want to check in there, especially if you are supposed to be somewhere else.
Also, I set the option on who can see my places to only friends and friends of friends. Although I use foursquare, I am selective on where I check in. And those places I could care two shits if anyone finds out about it.
Also, check out this non-option
So remember, your privacy is yours to control. Sadly, FaceBook made this an Opt-out option for overall Places control. You can find that in your privacy settings. So if you don't want to play in Places, you have to tell Facebook that. I'm sure this will change when the pressure hits.
UPDATE : Apparently the option to participate in places may or may not be on by default. I checked my account and my twins accounts and they were already enabled. For my wife, her's was set to OFF. Be curious as to what yours was set to when you checked.