If you have Visual Studio 2008 SP1 Beta installed make sure that you run the Visual Studio 2008 Service Pack Preparation Tool prior to the installation, Microsoft were trying to make this a seamless process, but on Heath Stewart's Blog he mentions that this was not possible in the end. The Preparation tool takes a while to run, but it is way better than having to uninstall everything :)
You will need your Visual Studio 2008 Install files when running the Preparation tool, but you should always keep the ISO handy any how…
A pure .NET 3.5 SP1 Betas install however (like out web servers) can just be updated by running the SP1 RTM install.
Required Downloads
Visual Studio 2008 Service Pack Preparation Tool
Microsoft Visual Studio 2008 Service Pack 1 RTM
Microsoft .NET Framework 3.5 Service Pack 1 RTM