Marcus Winters and I have done a few studies on the effects of Florida’s McKay voucher program for disabled students. These studies were published as Manhattan Institute reports:
“How Special Ed Vouchers Keep Kids From Being Mislabeled as Disabled,” Manhattan Institute, Civic Report No. 58, August 2009.
“Evaluating the Impact of Special Education Vouchers on Public Schools,” Manhattan Institute, Civic Report No. 52, April 2008.
It took a while, in fact a couple years, but a revised version of those two studies combined into one article has been published in the peer-reviewed journal, Educational Evaluation and Policy Analysis, which is the leading empirical publication sponsored by the American Educational Research Association.
You can read the full article on the EEPA web site here.
And here is the title and abstract:
Public School Response to Special Education Vouchers
The Impact of Florida’s McKay Scholarship Program on Disability Diagnosis and Student Achievement in Public Schools
Marcus A. Winters, University of Colorado at Colorado Springs
Jay P. Greene, University of Arkansas
Abstract
The authors expand on research evaluating public school response to school choice policies by considering the particular influence of voucher programs for disabled students—a growing type of choice program that may have different implications for public school systems from those of more conventional choice programs. The authors provide a theoretical framework to show that special education vouchers could influence both school quality and the likelihood that a school will choose to identify the marginal child as disabled. Using a rich panel data set from Florida, the authors find some evidence that competition from a voucher program for disabled students decreased the likelihood that a student was diagnosed as having a mild disability and was positively related to academic achievement in the public schools.
Jay P. Greene's Blog 
Now aren’t you ashamed of yourself, Jay, for rushing this research into print before it was peer reviewed? The process only takes a measly three years!
And I’m sure that Andy Rotherham and Sara Mead would wait for those three years before pushing their empirically unsupported claims that McKay increases special ed enrollments and hurts the achievement of disabled students.
And the AFT and PFAW and everyone else would also remain silent until the peer-reviewed evidence came in.