If you've created a new WPF project in VS2008 and then you try to design your UX in Expression Blend 3 you'll find that you get xaml view only.

What is required is to add

<ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>

to the project file.