If you have .NET Framework 3.0 SP1 installed on your machine, when attempting to install the Extensions - you'll get an error message saying:
Setup has detected that a prerequisite is missing. To use Visual Studio 2005 extensions for .NET Framework 3.0 (WCF & WPF), November 2006 CTP you must have the .NET Framework 3.0 runtime installed. Please install the .NET Framework 3.0 runtime and restart setup.
Now, apparently .NET Framework 3.0 SP1 is not .NET Framework 3.0 for the Extensions installer.
Seems like the best way to go around this problem is to fool the Extensions installer by putting a fake key into the registry:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{15095BF3-A3D7-4DDF-B193-3A496881E003}]
"DisplayName"="Microsoft .NET Framework 3.0"