From the Vista Team Blog

…
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.

– blogpost: Windows Vista SP1 availability for technical customers

And from my experience, it’s a problem which only has occurred late in the SP1 Beta cycle.  Didn’t see the problem with SP1 RC, but did with SP1 RC Refresh (6001).

The driver with the problem?  Well in my case, the Intel 2915ABG Wireless network driver.  I expected better from Intel.