The French have adopted Ubuntu as their primary systems. Stock markets now run on Linux. So does whitehouse.gov. Obama's campaign servers ran on FreeBSD.
It's not entirely unreasonable to see it changing. Plus, as you said, it would save the government plenty of money in licensing and support.
Maybe, it will save licensing costs, but it will not save on support costs. Contracting companies are not going to lessen their support bids for Linux compared to Windows. I wouldn't be surprised if the bid for support was higher to compensate for the lesser number of Linux-trained support people. It will probably be a wash.
Even if it is, the government would have easier access to and better control over an open-source stack than it would with Windows. It would also have a wider possible market because support vendors would not have to get Microsoft's blessing before providing services.
Actually, the US government probably could get Microsoft to make a change / add something a lot quicker than it could from the Linux community (money is a fine leverage). Many support organizations don't exactly get permission from Microsoft to bid.
Red Hat/Novell/Canonical could add/change Linux features just as easily as Microsoft could add to or change Windows. The Linux community isn't just a bunch of people who are writing code for the fun of it.
Not only that, but you can make Red Hat, Novell and Canonical compete against each other for the best solution.
As much schizophrenic as Microsoft's various divisions are, you can't make the Xbox division compete with the Office division on which one wins a contract for adding a feature to the Windows 8 kernel.
For better or worse, the US government has a lot easier time dealing with a central point of control and owner's of products are their institutional experience. Also, I gotta wonder how a change that was not popular would work if made by one of the distribution companies.
Also in this case the "desktop" would then be just any old client which could easily be upgraded/swapped out and something other than Windows could be used as OS.
I would be glad if this changed that, though. There's a potential for quite a bit of money to be saved.