http://blogs.pcworld.com/staffblog/archives/006433.html

"One small catch is that Microsoft will not offer the update to a small
subset of Vista PCs that have been found to have "problematic" device
drivers. Those drivers, Microsoft says, were not installed properly
initially and when Vista SP1 is installed OS problems ensue. For those
systems, "if Windows Update determines that the system has one of the
drivers we know to be problematic, then Windows Update will not offer
SP1," according to the Windows Vista Team Blog. "


LOL, I wonder how one incorrectly installs a device driver?