Don’t rename your .xcdatamodeld file

Ok, you can rename it but be prepared for weird bugs with Xcode 4.3 (and possibly earlier versions).  In my case I created a new Core Data model file and added it to an existing project.  Then I renamed the model file before attempting to build the project.  Yet even with a clean build and install (deleting the app from the device first), NSManagedObjectModel -initWithContentsOfURL returned nil even though my NSURL pointer was non-nil.

NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"ModelName"
    withExtension:@"momd"];
__managedObjectModel = [[NSManagedObjectModel alloc] 
    initWithContentsOfURL:modelURL];

Short answer: quit and reopen Xcode to fix the problem.

(Solution via Stack Overflow)