It seems that moving System.Runtime.CompilerServices.ExtensionAttribute to mscorlib breaks StructureMap Scans of Namespaces containing ExtensionMethods and possibly other IOC scenarios.
Unless structuremap is explicitly told to ignore Attributes, it evaluates any attributes on the class being scanned. It tries to examine them using reflection and looks as though the system reflection classes try to load the ExtentionAttribute from the mscorlib assembly where that attribute would be at compile time but not at run time if the code was compiled on a .net 4.5 install and run on a 4.0 install.
I am able to work around this by adding y.IgnoreStructureMapAttributes(); to my scanning rules which tells StrctureMap not to worry about analyzing the Attributes on classes.
Due to the popularity of Structuremap and that this may occur in other Reflection based scenarios with analyzing the attributes of a class containing Extension methods, this may be a fairly serious compatibility issue. It only took me one day of trying vs11 beta/.net 4.5 to run into this.