If you had Visual Studio 2008 RTM (or a Beta of SP1) already installed on your machine, the installation of SQL Server 2008 requires to install the final SP1 build of Visual Studio 2008 as a prerequisite (explained in more detail here).  The good news is that Visual Studio 2008 SP1 and .NET Framework 3.5 SP1 are now available as well.

On a related topic, SQL Server 2008 Express is available now for download.  Details about the feature sets of the current Express Edition and the two upcoming, further enhanced Express Editions can be found in this posting on the Data Platform Insider blog.