This article finds that most employer-sponsored health insurance plans provide mental health coverage but at lower levels than for other medical benefits.  The authors conclude that parity legislation is effective but limited.  They predict that legislation is unlikely to yield universal employer-based parity citing, among other things, the decline in mental health coverage among small businesses between 1997 and 2003.

Teich, J. L., & Buck, J. A. (2007). Mental health benefits in employer-sponsored health plans, 1997-2003. Journal of Behavioral Health Services and Research, 34(3): 343-348.  DOI: 10.1007/s11414-006-9050-2.  http://www.springerlink.com/content/y2121502144268w1/