If your solution to having uniqueIdentifier deprecated was to use the MAC address of the device in some way then you're going to have a bad time in iOS 7. […]
The message here is very clear, Apple are not afraid to remove any API that makes tracking users across apps by different vendors possible.
If your apps use the MAC address to identify an iOS device, the system will return the same static value for all devices running iOS 7.
It is a bad week for advertisers (see: Google Play new policies).