Found a book: Deconversion: Qualitative and Quantitative Results from Cross-Cultural Research in Germany and the United States of America. It's recent (2011) and seems to be the best research on the subject available right now. Does anyone have access to a copy?
There's a PDF (legal, even!) here, linked next to "download".
See also their website/theologie/forschung/religionsforschung/forschung/streib/dekonversion/), which is probably more digestible.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.