This has been a hot topic in 2013. To date, eight states have laws which protect employee privacy, re: personal passwords (Arkansas, Colorado, Nevada, New Mexico, Oregon, Utah, Vermont, and Washington) and several states have pending legislation.
For example, in Georgia:
Status: Jan. 30, 2013; In House. Read second time. Adjourned. Carryover.
Prohibits employers from requesting username, password, or other means of accessing an account or service for the purpose of accessing personal social media through an electronic communications device of employees or prospective employees with certain exceptions, provides for penalties, provides for related matters, repeals conflicting laws.
For a list of states and their status, see
Employer Access to Social Media Passwords 2013 Legislation