Installing WCF CTP Extensions with VS 2005 after .NET 3.0 service packs....
If you're installing Visual Studio 2005 and need the WCF CTP Extensions, BUT you've already installed .NET 3.0 with service packs, the installer fails.
To override the check use the following:
msiexec /i vsextwfx.msi WRC_INSTALLED_OVERRIDE=1