As expected, Microsoft has released Windows Vista SP1 for download.
It's available for download from the Microsoft Download Center in both 32-bit (435MB) and 64-bit (727MB) flavors, and via Windows Update (which will be a much smaller download, something in the region of 65 to 70MB).
It won't be installed automatically on your system just so if you want it you'll have to check for it manually. If over the next few hours you don't see it, something might be preventing installation. Microsoft has a knowledge base article (KB948343) which looks at the possible reasons for this.
Also worth noting is that if SP1 hoses your system, Microsoft will provide you with free tech support to get you up and running again.
My ZDNet blogging colleague Ed Bott has more information on SP1 here.