I'm also very, very interested in a career as a mental health services researcher / mental health policy analyst. (Right now, I'm in the middle of doing an MPH, which I love.) It seems that both the NIMH and the New Freedom Commission report that came out a few years ago both emphasize this area as an important priority. Apparently, while there's been a lot of progress in finding good, efficacious treatments for mental illness over the last 15 years or so, we've got a long way to go before they're actually implemented properly. A huge part of the problem is the uber-fragmented, state-based mental health care system. So I'm crossing my fingers that this will be a growing field, and that lots of research $$$ will be directed toward it in the near future.
BTW, Penn has a Center for Mental Health Services Research/Policy. Anyone know anything about it, reputation, etc? I think they also offer 2-year fellowship.