Using Apple's preferred method of generating a CFUUIDRef is probably the better solution as I feel the MAC address may be removed in the future as well. If you use this and save it to your NSUserdefaults you will have it persist unless the user deletes the app. If you want to have a generated unique ID that you can share between apps or persist between installs you should look at using the UIPasteboard and saving your generated UID with a key that you share between apps.
//Create a unique id as a string
CFUUIDRef theUUID = CFUUIDCreate(NULL);
CFStringRef string = CFUUIDCreateString(NULL, theUUID);
//create a new pasteboard with a unique identifier
UIPasteboard *pasteboard = [UIPasteboard pasteboardWithName:@"youruniquestring" create:YES];
[pasteboard setPersistent:YES];
//save the unique identifier string that we created earlier
[pasteboard setString:((__bridge NSString*)string)];
//accessing it
UIPasteboard *pasteboard = [UIPasteboard pasteboardWithName:@"youruniquestring" create:NO];
NSLog([pasteboard string]);
I have written a brief tutorial here but its basically the lines above: http://www.roostersoftstudios.com/2012/03/26/a-way-to-share-data-between-apps-on-a-given-ios-device/