x64 Vista Home Premium here, no SP1 even after reboot and about 10 re-checks for updates. Looks like this might be a Business/Ultimate only deal?
Not quite. It seems very random at, perhaps at first glance. My friend has Ultimate 64-Bit also and he got it earlier. Lots of premium users have gotten it the same. I think it's more likely that they have some queuing system in place so it doesn't bog down their system. Or there is some sort of driver/patch that triggers it.