Removing certain referenced assemblies from a Silverlight 4 Application project in Visual Studio 2010 means they can't be readded and therefore the project will never compile.
It seems that if you remove the reference to mscorlib or System.Core (may be others too?), which Visual Studio processes just fine, if you then try to readd the references, they do not reappear as references. There is no visible error, they just aren't referenced by the project.
As a consequence the error "Predefined type 'System.Object' is not defined or imported" is reported by the compiler. This can only be resolved by adding the reference by manually editing the csproj file: closing/reopening/refreshing/cleaning the project doesn't seem to have any effect.