If a user does not have select permissions on another users�??s feature classes or object classes, then the first user will not see these objects listed in ArcCatalog or when using Add Data. However, what they will see are Feature Datasets and Relationship classes, and special classes, like the Geometric Network (things that are not really database tables). However, this is post 9.2 functionality since at 9.2 you would see everything listed in the GDB_ObjectClasses table, regardless of permissions.
Jeff DeWeese
Esri Enterprise Systems Architect