First and foremost, what possible purpose can it serve to use a MAC address for any part of a user interface? Unless your target market is networking professionals, this is an incredibly obscure piece of information that means little or nothing to most users.
Many networking devices (such as Cisco wireless routers) support spoofing MAC addresses. Many Asian knock-off manufacturers (who clone hardware from other vendors) also reuse the original vendor's MAC addresses. Learning to spoof a MAC address is one of the first steps to becoming a garden grade hacker. Relying on the uniqueness of MAC addresses is fairly safe for most users, but no where near "iron clad" for these reasons and other reasons too.
The average end user has no clue what a MAC address is. You need to be at least moderately geeky to understand what a MAC address is, much less how to determine what your MAC address is. Since many machines have multiple MAC addresses, it is tricky to get users that can find a MAC address on a machine to consistantly provide you with the correct MAC address.
-PatP