The CPRI has released the results of its study about the numbers of "orphan" Greek and Roman artifacts in private hands. See http://www.cprinst.org/Home/issues/project-on-unprovenanced-ancient-objects-in-private-us-hands In addition, the CPRI's American Antiquities Review has also provided some helpful background information. See http://www.cprinst.org/american-antiquities-review
The study has already attracted comment from archaeological bloggers David Gill and Larry Rothfield. See http://lootingmatters.blogspot.com/2009/11/new-report-on-private-collectors-in.html and http://larryrothfield.blogspot.com/2009/11/orphan-antiquities-study.html
Though Gill has been particularly critical, he has not offered his own estimates of the number of "orphan" Greek and Roman artifacts in private hands or his own proposed methodology for estimating the number of such artifacts.