Hi ax12,
I'm a recently admitted HKS MPP student still in the decision process. This document may answer some of your questions -- HKS's 2013 Employment Outcomes: http://www.hks.harvard.edu/var/ezp_site/storage/fckeditor/file/pdfs/degree-programs/oca/employment_overview.pdf
Hope it helps!
Congratulations all!
I've been told by several students that IDEV is by far the largest concentration (though it is capped). One of them also indicated that the next two largest concentrations are Energy/Environment and Latin America/Caribbean.