It is here, well the Release Candidate is. Go to http://technet.microsoft.com/en-us/windowsvista/bb738089.aspx for information on how to download it.
Multiple download options are avaliable on that site quoted above, you can download a small patch which basically adds Service Pack 1 to Windows Update in Vista (a few prior updates were required which are like sp1 precursors), you can also download stand-alone packages or through TechNet assuming you have a subscription.
The whitepaper which contains an overview of changes in service pack 1 is avaliable here.
For a more in-depth look at the changes made between Vista and Vista SP1 check here.
To see the Vista SP1 release notes go here.
To view a list of hot fixes included in SP1 check here
I will note some other handy links and details about Vista SP1 when they become avaliable. Check back soon!