Good question, isn't it? Although Microsoft has listed a set of drivers that will prevent you from being offered SP1, it's not always clear why a user can't get it.
The service pack is being withheld from machines containing one or more of the listed drivers, because, as Microsoft put it in a support document, "these device drivers are problematic on Windows Vista-based computers when you update to Windows Vista SP1."
That made an industry analyst wonder about Microsoft's driver-testing process. "When Microsoft said there were problems with drivers, I assumed it was some odd scanner or camera, or an ancient printer or something," said Michael Cherry, analyst at Directions on Microsoft, a Kirkland, Wash.-based research firm. "But then I saw the list. It makes me wonder what's going on with device driver testing.
"Microsoft keeps saying that there's this vast ecosystem of device drivers, but it appears there's a much smaller number of reliable, well-tested drivers. Because if these drivers [on Microsoft's list] were tested, that calls into question the testing process."
Despite the so-called WHQL driver certification process, it's clear that some questionable drivers have made it through the process. As people who have been deeply involved with WHQL in past lives, we can tell you, it's actually not surprising.