Silverlight 5 is RTM and it looks like the version bump has broken Cecil.
at Mono.Cecil.BaseAssemblyResolver.GetCorlib(AssemblyNameReference reference, ReaderParameters parameters)
at Mono.Cecil.BaseAssemblyResolver.Resolve(AssemblyNameReference name, ReaderParameters parameters)
at Mono.Cecil.BaseAssemblyResolver.Resolve(AssemblyNameReference name)
at Mono.Cecil.DefaultAssemblyResolver.Resolve(AssemblyNameReference name)
at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
at Mono.Cecil.ModuleDefinition.Resolve(TypeReference type)
at Mono.Cecil.TypeReference.Resolve()