My experience is primarily DoD, which is the bulk of the government's spending on this sort of thing. For enterprise spending, Apple doesn't get much love, outside a general trend towards iPhones for "company" phones, but even that's rare, with people using their own phones instead of being issued one a lot of the time (they can get their plans paid for or subsidized if the phone is a requirement of the job). Labs often end up outside a lot of the enterprise type procurements and so have a bit more leeway in that regard, but while they spend a lot of money, it's still a drop in the bucket for Apple.

I do agree with your general point.

Your characterization ("Maybe some labs that wanted a unixy OS and, again, convinced their purchasing officer to buy a Mac Pro?") is a little off -- where I work, MBP's for laptop replenishments are treated exactly the same way as Windows systems, you just tick a different button on the order form.