The company I work for, a household name known for its outstanding technologies, has banned Vista and IE7 from the campus thus far. OS X has been banned since its inception except for DTP and PR applications. Mortal IT people may not connect their Macs to the network.
There is a certain distrust of powerful machines in corporate security circles. There are paranoid people everywhere.
I've only been permitted to use XP since 2004.
It'll be a while before MS recoups its investment in Vista.
It is the rule, not the exception, that companies do not immediately buy new Windows updates whole hog and migrate to these new systems as soon as they are released. There is usually not a compelling business reason to do so and the benefit would not outweigh the cost. It is also the rule that they use the same system company wide. "Banned" sounds like a dubious word for what your company has done. More likely, they made a business decision not to use Vista at this time, and have a policy that people cannot simply install any software they like on their computers.
If I were running a company, I would do the same thing.
Having said that, I would allow Mac OS upgrades to be installed within months of new release, and definitely prefer it to Windows.