Update (08/11/2008): See this blog post, which explains the issue with the RTM version of SQL Server 2008 and VS2008 SP1.
There seems to be an installation issue with the RTM version of SQL Server 2008, if you already have Visual Studio 2008 installed on the same system. The RTM version of SQL Server 2008 includes the Visual Studio 2008 SP1 bits for BIDS (Business Intelligence Development Studio) and SQL Server Management Studio, which both use Visual Studio components. Both of these are part of the Tools install components in SQL Server.
The issue is that Visual Studio 2008 SP1 is not yet widely available, so you cannot upgrade Visual Studio 2008 before installing SQL Server 2008 as the installer will tell you to do. Based on this the release of Visual Studio 2008 SP1 must be coming very soon, otherwise the SQL Server team would not have included it in the RTM version of SQL Server 2008.
See this Microsoft Support KnowledgeBase entry for more details and some possible solutions