I'm currently in the process of migrating a (big) custom development from NW 7.0 to 7.5 and fighting with compile errors.
The IUser interface got a new method that returns I18NFormatterFactory according to http://help.sap.com/javadocs/NW75/SPS06/CE/se/com.sap.se/com/sap/security/api/IUser.html.
Is there a public part I can reference for this? I guess not. I'm really astonished how an obviously bad design decision like this could ever make it into a release. IUser is public and if internal methods are needed in an interface, move it to a dedicated or derived one.