It's part of The Mental Health Parity and Addiction Equity Act of 2008
Here is a fact sheet from the Dept of Labor
http://www.dol.gov/ebsa/newsroom/fsmhpaea.html

At my company, it meant that co-pays became the same for medical and mental health visits, went from $25 per visit to $15 per visit in 2010.

Yes, it applies to firms with more than 50 employees, MUE. But, they cannot change how you are covered in the middle of the plan year.