The various bits for .NET Framework 3.0 RC started being released on Monday. They are all available on the download servers now so here's my install list for setting up a development environment for .NET Framework 3.0 RC. There is also an updated Go-Live license available for this release and you are recommended to upgrade.
You may wish to read the associated README which includes important uninstall instructions for previous builds of .NET Framework 3.0. We also have a guide to migrating code from WF Beta 2.2 to WF in this RC.
I have a Windows Vista RC machine and a Windows XP machine which I've installed these on. It's also the same install on Windows Server 2003 if you use that as your development environment.
Finally, check out samples on the community site for .NET Framework 3.0 and read what we have on MSDN.