Vista SP1 has landed, but not for everyone. Those using a 64-bit installation of Vista may have noticed a surprise courtesy of Windows Update. Microsoft has released the 64-bit version of SP1 ahead of the 32-bit release.
There are still very few users actually making use of a fully 64-bit operating system. If you are, you are likely in the minority of users.
via a post at GRC.com newsgroup. A user with Home Premium x64 and Business x64 of Vista confirmed Vista SP1 was offered via AU.