Windows Vista SP1 does not appear in Windows Update

Hi experts,

I read that Vista SP1 was out. so I rushed to windows update only to find it wasn't showing up. Is this because Microsoft has deemed it incompatible with my hardware? Or is it a windows update bug? If it helps, the computer is a Lenovo X61t. I know I could always download it manually, but if it is incompatible I would rather not.

Many thanks!
-Max