MAC address only _have_ to show up on the local lan (wireless or wired). If you are talking to something through a router, you don't see the MAC address, just the IP address.
However, Apple devices tend to use the MAC address as a device identifier (it is after all a unique value for each device), and they make extensive use of that unique value in their apple proprietary infrastructure.
One of the 'hacks' that has happened 'recently' was someone noticed that accessing iDevice info on the ATT website used the MAC address in the URL, changing the MAC address provided gave you access to other people's information.
I don't know anyone other than Apple that does this same thing.