From the changes on going to J2SE (which appear to be implemented in JSE 6.  Maybe this relates?

Class Loader - Previously, it was possible to specify a non-binary class name to ClassLoader methods that take a String class name argument. This unintended behaviour was not compliant with the long-standing specification of class names. As of 5.0, parameter checking of these ClassLoader methods has been modified to comply with the specification, and any class name that is not a binary name is treated like any other unrecognized class name. Since the APIs that explicitly require or return class names (for example, Class.forName or Class.getName) use the binary name for reference types, it is unlikely that the typical user would have produced a class name that would have returned a Class.

Jonathan