What do you guys think of this ... reports that it's becoming more common for prospective employers to require applicants to turn over their social media passwords as part of the interview process?

The employers say it's a good way to weed out people that might be involved in illegal activities (I guess they mean drugs) and such.

Good, or bad? If you think it's good - do you think there's a line that shouldn't be crossed ... or would it ultimately be OK if they required passwords to email accounts and such as part of the interview process?