Just had this issue: Opening a project created on another machine threw an error in Visual Studio 2010. The output window contained this message:
The imported project "C:\Program Files (x86)\MSBuild\Microsoft\Silverlight\v4.0\Microsoft.Silverlight.CSharp.targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.
The problem is that I didn’t have the Microsoft Silverlight 4 Tools for Visual Studio 2010 installed. This can be downloaded from here: http://www.microsoft.com/downloads/details.aspx?FamilyID=eff8a0da-0a4d-48e8-8366-6ddf2ecad801&displaylang=en