Eh. Most companies and organizations that require a reliable infrastructure usually hold off on upgrading to the newly released operating systems until they have a chance to analyze it, test their applications on it, and get the security and support infrastructure in place for it. This is standard practice with most large businesses, especially when the new OS is known to break compatibility. Even if they wanted to deploy it immediately, it is very unlikely that most large companies would have the time or resources to devote to testing and migration simply because Microsoft launched a new OS.
For example, I did some contract work for a major insurance carrier here in the states building a test environment for their IT desktop infrastructure. Back in the 1998-1999 time frame they were transitioning their desktops from OS/2 to Windows NT4. Then I worked with them again in 2002 when they were evaluating Windows 2000 and Windows XP. They had a lengthy process to evaluate both platforms and test their applications for compatibility before they decided which one to adopt and how to go about it. Application compatibility testing (and re-testing) took almost 6 months. It was expected to take almost 3 years before all of their desktops would be replaced with the new OS.
I assure you, the move from NIST is nothing new. It's also funny that the article that you referenced actually points says "NIST.gov must be aware that their decision is likely to draw attention and be misconstrued." So it sounds like it hasn't actually been "banned", but that they're not allowing it to be deployed until they've tested it. No surprise there.
I wouldn't read too much into Dell's decision either, because when Windows XP was released you were still allowed to choose between XP and 2000 for awhile from most vendors. This is at least partially attributable to companies being reluctant to deploy untested technology.
Seeing as they started a SP for it a week after it was out, it looks like it may have needed to be cooked a little longer before they put it on the table for people to take.
This is pretty standard practice too because software development is a long process. A service pack can take 9 months to a year to develop and test properly (they go through Beta testing too), and invariably there are bugfixes added to the patch over the course of development. If they want to release a major patch within a year of OS release they pretty much have to start immediately.
Also, keep in mind that service packs are not solely bug fixes. A lot of times they implement new features/functionality or upgrades to existing functionality. A prime example would be Windows XP SP2 bring the improved security and firewall, or one of the Windows NT4 service packs that added support for larger disks. Considering that Vista had some features that were in earlier Betas but didn't make the release code, it's entirely possible that some of them could be included in service packs yet to come. And in that case they probably actually started working on it before Vista was released.