There was a MOMC_NO_INVERSE_RELATIONSHIP_WARNINGS setting on Xcode 3 but I cannot find it in Xcode 4.2 and trying to set it as a user defined build setting have no effect.>
The setting is still there in Xcode 4. At least it is for me when I create a new project in Xcode 4.2 that uses Core Data. The MOMC_NO_INVERSE_RELATIONSHIP_WARNINGS setting is in the Data Model Version Compiler (MOMC) Warnings collection, which is towards the bottom of the build settings list.