Big sales for Microsoft
The United States Army has announced the decision to upgrade hundreds of thousands of PC desktops from Windows XP to Windows Vista by 2010. This undoubtedly will make Microsoft happy, as sales of Vista were presumed to be never as high as they had hoped.
The U.S Army will also be upgrading all copies of Windows Office 2003 to Office 2007. Instead of waiting for Office 2010, it looks like the Army is choosing to go with Office 2007 out of security concerns.
This switch over to Vista will be one of the largest OS mass-upgradings to happen in the United States. While Vista has made its way into many large corporations and organizations, there has been some marketplace reluctance, and Windows XP remains a very common sight, 8 years after release.
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In a word, cost. Windows 7 is much more polished and refined version of Vista, but at a higher cost. Going with Vista will cost less than going with Windows 7.
I don't think it's a reasonable decision to proceed with Vista now, when by 2010 a superior option will be available. I hardly think compromising military security and functionality is worth compromising by not holding off on upgrading for 6 months.
Regarding Win 7 versus Vista ,I would assume that Win 7 is just too new to be considered for this upgrade. Win 7 probably will be more secure than Vista with a bit of work, but any totally new operating system is just not 'experienced' enough to be really be trusted to run such a security-conscious network .
And, on the other hand, the U.S. Army is probably like any other large bureaucracy where it takes years just to get balls rolling on things, so they probably planned this upgrade years ago, when Win 7 was far too much of an unknown to bank on.