This is the code I'm using to call the people picker, but the prompt label text doesn't change:
ABPeoplePickerNavigationController *picker = [[ABPeoplePickerNavigationController alloc] init];
picker.peoplePickerDelegate = self;
picker.displayedProperties = [NSArray arrayWithObjects: [NSNumber numberWithInt:kABPersonEmailProperty], nil];
picker.navigationItem.prompt = @"Choose a contact to...";
[self presentModalViewController:picker animated:YES];