Request for some help -PC buying stats

GCN ( http://www.gcn.com/print/25_7/40263-1.html ) is running an interesting series on desktop Linux. One of the many questions it raises, but doesn't address, is how big the market is. Does anyone have access to real numbers? for DOD, for larger contractors, for the federal gov't? for its agencies? (all US, of course). I know there are surveys around, I just can't find them.. help! ( Please? ) Thanks!