The relevant point is not what they chose but that a choice exists.
Interestingly at my place we have a choice (Red Hat/CentOS, OS X, Windows XP, Solaris, Windows 7) and people, even full-time Unix sysadmins, are choosing Windows 7. Make of that what you will.
My first guess would be that the admins are using their desktop boxes mostly as dumb terminals to the unix servers, and chose Windows because of easy integration with email and calendaring.
As to why Windows 7 instead of XP, maybe it's getting good word of mouth? Or maybe getting a fresh install of XP is more of a pain than going with Windows 7, and they just can't be bothered?
From my perspective, that list (and the fact people choose Win7) means:
1. Red Hat / CentOS is better than Ubuntu, but if that's my only free/open source Unix-like option, it sucks.
2. MacOS X is good for the "tell me how my working environment should look" crowd, I guess.
3. Windows XP and Windows 7, but not Vista? Good! Too bad XP and 7 only look good by comparison.
4. Solaris for workstations. Damn, this company is interested in giving people options! Too bad I'm not terribly sold on any of the options they offer.
5. I wonder if people are choosing Win7 because it's better, or because it's "new" and still "Microsoft" (and thus "better", but not necessarily better).
I'm a bit of a cynic when it comes to technology options and how people make decisions, I guess.
Interestingly at my place we have a choice (Red Hat/CentOS, OS X, Windows XP, Solaris, Windows 7) and people, even full-time Unix sysadmins, are choosing Windows 7. Make of that what you will.