This is, aside from the additional cost (which will, of course, pale in comparison to other war expenditures), a great move for the Army. It's true that Macs are dramatically safer than their Windows counterparts.
But the more we read about Apple making up ground in market share against its PC competition, it begs the question: How long can Mac users go without worrying about security. Seeing the Army deploy Macs won't help matters.
Sure, Macs are built on the inherently more secure Unix framework - but there are vulnerabilities in everything.
When will the hackers' interest reach the tipping point?
It's the million-dollar question - and unless Macs receive widespread deployment in corporations and other government entities - it may be never come. But you better believe that the hacker community isn't sitting back, watching the PC vs. Mac commercials preaching Apple security, without craving the opportunity to prove them wrong.