In my blog post on the subject, I noted that we were going to make SP1 available to customers in stages to make sure we delivered a great experience.  As I explained, one reason for this is that we are working through an issue with a small set of hardware devices that may not function properly after the Windows Vista-based PC they are installed on is updated to SP1.  This is an issue with the way the device drivers were re-installed during the SP1 update process, not with the drivers themselves -- these drivers worked on Windows Vista RTM and they work on Windows Vista SP1.  For new PCs with Windows Vista SP1 pre-installed, this is not an issue. 

We are working with the manufacturers of these devices to get the drivers and their install programs updated, and also working on other solutions we can use to ensure a smooth customer experience when updating to SP1 over Windows Update.