Windows Vista SP1 is released to Windows Update ... as an optional update (since march 18th). It should be available as an automatic update around mid-april. The Windows Vista Team explained all that in there blog.
If you have a WSUS server in your organisation you could deploy SP1 now by importing the update from Microsoft Update Catalog. The WSUS team has a post describing the procedure in there blog.
You should however note that you can't import an update in WSUS from the Microsoft Update Catalog if your server is setup as a downstream server. You will receive an error number 80131509.
