The American Academy of Sleep Medicine has limited physicians to being medical director of no more than 3 sleep centers/labs: http://www.aasmnet.org/Resources/PDF/Dec2008CenterStandards.pdf
This requirement is being phased in and appears to take effect July 1,2009.
update (2/14/09) It has been clarified by the AASM on their discussion forum that this requirement takes effect July 1, 2009. A physician is limited to 3 medical directorships and/or board-certified sleep specialist positions (the combination can not exceed 3). A PhD. sleep specialist is limited to 3 board-certified sleep specialist positions.