If what I've read is true, lots of places. A school teacher in my area just got fired for making a student give access to his/her Facebook account which the teacher used to punish students who had talked about the teacher.
It's apparently quite common. The real kicker is when they also include a non-disparagement agreement in the hiring process, so that they can easily fire you for non-publicly posting about your job.
I've read about police and city agencies requiring social networking passwords to be given up. Same for departments of corrections. Here's a photo of a job application for a clerical position at a police dept: http://i.imgur.com/hWsZT.jpg (From this reddit thread: http://www.reddit.com/r/WTF/comments/mtenb/wife_came_across_...)
It's apparently quite common. The real kicker is when they also include a non-disparagement agreement in the hiring process, so that they can easily fire you for non-publicly posting about your job.