Now that you can get Visual Studio 2008 SP1 RTM, you probably want to rush out and install it. I would hold off on that until you read Bob Ward's blog entry, which explains the situation, and should help clear up some confusion on whether you need to install anything:
SQL Server 2008, Visual Studio 2008 SP1, and .Net Framework 3.5 SP1 explained....
For those that are really in a hurry, you can get the core download here (this is the base installer which will download the files you need at runtime, based on operating system, edition, etc.):
http://www.microsoft.com/downloads/details.aspx?FamilyId=FBEE1648-7106-44A7-9649-6D9F6D58056E
And as Joe points out, if you want the whole download (800+ MB), you can get that here:
http://www.microsoft.com/downloads/details.aspx?FamilyId=27673C47-B3B5-4C67-BD99-84E525B5CE61