On two computer systems, I have fully legal and validated Vista 64-bit installs. On both machines, I use the Windows Update system to ensure I have all the latest patches and updates. Both machines pass Microsoft's "Genuine Advantage" checks. In both cases, the machines are running service pack 1. According to Wikipedia, Microsoft released Service Pack 2 for Vista on April 28, 2009, and it was released to Windows Update on May 26, 2009. Neither computer system is "seeing" SP2 as an available upgrade. Why not?
Both systems have seen more recent critical and important updates, and I have applied these. On one system, I downloaded SP2 and applied it manually. The other system was assembled and had Vista installed in the past month, but still did not see SP2 as an available update (the Vista disk was an OEM version with SP1 integrated).
Update: Strangely enough, after posting this article, SP2 was pushed out by Windows Update to the machine which did not already have it installed. Also, to a laptop I had forgotten about. I'm not sure why Windows Update had this change of heart, perhaps Microsoft reads Superuser.com. :)