Alternative to Mac address as unique identifier

With the release of iOS 7, Apple is eliminating access to the MAC address. Requests for the Mac address will return an address of 02:00:00:00:00:00.

Apple recommends developers use either identifierForVendor or advertisingIdentifier

One precaution with identifierForVendor:

The value changes when the user deletes all of that vendor’s apps from the device and subsequently reinstalls one or more of them.