See Incorrect solution build ordering when using MSBuild.exe at The Visual Studio Blog:
Follow this principle: do not use dependencies expressed in the solution file at all! Better to express dependencies in the file that has the dependency: put a project reference in the project, instead. In our example, that would be a project reference from B to C.
You may not have done that before because you didn’t want to reference the target of the project reference, but merely order the build. However, in 4.0 you can create a project reference that only orders the build without adding a reference. It would look like this – note the metadata element, and all this is inside an <ItemGroup> tag of course:
<ProjectReference Include="foo.csproj">
<ReferenceOutputAssembly>false</ReferenceOutputAssembly>
</ProjectReference>
Note that you have to add the child element with a text editor — Visual Studio can add a project reference, but doesn’t expose UI for this metadata.
I can tidy up by removing the dependency in the solution file as well – removing now-unnecessary lines like this — your GUID will be different, but use the VS dialog and it will do the job…