Unique identifier for an iPhone app

For an iPhone app that submits images to a server I need somehow to tie all the images from a particular phone together. With every submit I'd like to send some unique phone id. Looked at

  [[UIDevice mainDevice] uniqueIdentifier]

and
  [[NSUserDefaults standardDefaults] stringForKey:@"SBFormattedPhoneNumber"]

but getting errors in the simulator.

Is there an Apple sanctioned way of doing this?

What errors are you getting? [[UIDevice currentDevice] uniqueIdentifier] (edited to fix API, thanks Martin!) is the officially recommended way of doing this.

You can also use CFUUID to generate a UUID. Here's some code:

NSString *uuid = nil;
CFUUIDRef theUUID = CFUUIDCreate(kCFAllocatorDefault);
if (theUUID) {
  uuid = NSMakeCollectable(CFUUIDCreateString(kCFAllocatorDefault, theUUID));
  [uuid autorelease];
  CFRelease(theUUID);
}

By far the easiest and most appropriate way to obtain a unique identifier is to use the mechanisms Apple explicitly provides for obtaining one - [[UIDevice currentDevice] uniqueIdentifier]. You can not guarantee that the phone number will be unique to the device or that the device will even have a phone number. Beyond that, doing so is a horrible idea as it is a definite invasion of the user's privacy. Even the uniqueidentifier should be hashed if you are going to store it in any way.