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.