It’s out:

http://www.microsoft.com/downloads/details.aspx?FamilyId=FBEE1648-7106-44A7-9649-6D9F6D58056E&displaylang=en

Make sure to read the setup notes on the download page. For more information on SP1, see here:

http://msdn.microsoft.com/en-us/vstudio/products/cc533447.aspx

UPDATE: If you had a pre-release version of SP1 installed, you’ll want to run the Visual Studio 2008 Service Pack Preparation Tool.