For each iPhone, MAC address is unique ( except for iOS Simulator ). You can obtain the information to identify the device. This question will guide you to get MAC address. However, iOS7 disallows use of MAC address. Therefore, for iOS 6 or before, you can use MAC address; for the coming iOS 7, you can use the following method.
Since iOS 6, Apple suggests to use Advertising Identifier, which can be found in ASIdentifierManager class. Example code:
NSUUID *uuid = [ASIdentifierManager advertisingIdentifier];