1. Login to your Facebook account
2. At the top right select Account -> Privacy Settings
|While better that Everyone, "Places" is still enabled for Friends|
3. Select Customize settings near the bottom
4. Scroll down till you see Places I check in - change accordingly (lock down to only me or say certain trusted friends - read: not the 500 people you added for Mafia Wars) - you'll also want to leave Include me in "People Here Now" after I check in unchecked.
|Only Me is the safest, you decide who you want to share with|
5. Further down you will see Friends can check me in to Places - by default it's set to Select one this sounds a bit too vague to me, given Zuckerburg's history with protecting my privacy, I'll go ahead and change this to disabled.
|I don't think so...|
There you go, all done, and hopefully safe until the next sneaky, notice-less Facebook update or release.