NSFetchedResultsController crashing on performFetch: when using a cache

I had a similar problem with one of my apps, when the Apple released the new iOS 4.0.
Search:

fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[self managedObjectContext] sectionNameKeyPath:nil cacheName:nil];

And set the value of the parameter cacheName to nil. It worked for me, hope it will for you. Let me know.