This error can also occur when opening a Silverlight project that was built in SL 4, while you have SL 5 installed.
Here is an example error message: The imported project "C:\Program Files (x86)\MSBuild\Microsoft\Silverlight\v4.0\Microsoft.Silverlight.CSharp.targets" was not found.
Note the v4.0.
To resolve, edit the project and find:
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
And change it to v5.0.
Then reload project and it will open (unless you do not have SL 5 installed).