If you are using Visual Studio 2008 and blend together you have likely ran into the problem with loading a VS2008 created WPF project into Blend. It throws up an error about an invalid project file and probably complains about the following line:

<Import Project=”$(MSBuildToolsPath)\Microsoft.CSharp.targets” />

Fortunately there is a service pack out for Blend that fixes this problem (and some other odd issues with Blend). You can download it here: http://www.microsoft.com/downloads/details.aspx?FamilyId=A07196D1-971B-4710-99C8-D3D9603CCFCB&displaylang=en