Right, there is just a .csproj file which will get upgraded if opened in VS2010 (and a .sln file created if saved).
The reference is not path-specific and looks something like
Code:
<COMReference Include="VisBuildSvr">
<Guid>{AD2C35D1-C804-4CCA-8031-236A23427EB0}</Guid>
<VersionMajor>1</VersionMajor>
<VersionMinor>0</VersionMinor>
<Lcid>0</Lcid>
<WrapperTool>tlbimp</WrapperTool>
</COMReference>
I believe VS will recreate the interop DLL if needed.
Still need to experiment with .NET framework versions, will bump up on the priority list (don't recall if that limitation was specific to actions that display a UI).