With iOS 6 Apple has introduced a new property in the UIDevice family, similar in nature to the deprecated UDID. identifierForVendor is an alphanumeric string that will uniquely identify a device to the developer.
All applications on the device by the same developer will return the same alphanumeric string.
One important note, if you delete all your apps on a device the next time you install one or more of your apps, there will be a new value created.
Here is how to get the current identifierForVendor value: