SQL Server MVP Phil Brammer has already blogged about the fact that, if you have Visual Studio 2008 installed, you have to install VS 2008 Service Pack 1 before installing SQL Server 2008 "Katmai". Why? Because Katmai contains bits and pieces of SP1, and will refuse to install over VS 2008 RTM.
(Just to make life more interesting, Microsoft didn't release VS 2008 SP1 until several days after SQL Server 2008.)
You can read a good blog post that summarizes the entire issue, and explains the dependencies, here, by SQL Server support engineer Bob Ward of Microsoft.