Using the MAC as an ID is a flawed method. Devices come and go, MACs change (because the NIC changed, etc.) Ultimately -- and here's the end of it -- you'd have to be maintaining a registry of every device's MAC within your network to know what's what, where, and who's using it. Just gleaning a "list of mac's" is simple enough, albeit a tedious pain in the ass: arp tables, cam tables, watching broadcast traffic, etc. Knowing the location of each device gets a lot more tricky; sure wired devices will be tethered somewhere off a switch port, but that's not always enough. Wireless devices, on the other hand, can be anywhere. And none of that addresses who's device it is, who's using it, and exactly what it is. (OUI's only go so far, and assume no one is forging their MAC.)
When someone carries a device into the office, does something questionable with it, and then leaves, tracking the who/what/where/when becomes much harder. When the questionable act isn't mentioned for weeks or months, it can become impossible.
Case in point, I had to trace down a "hacker box" in the network. It was a lot easier because it was ON and ACTIVE at the time. Looking through cam tables, I could only go so far as an unmanaged hub. (yes, this company still used far too many hubs. even in 2003.) Careful inspection of the hub (and unplugging one port at a time), localized it to a desk. An unoccupied cubical, in fact. By the time I got to that desk, the machine had been turned off (it was still warm), the "warez" hard drive had been removed (windows registry remembers it), and the non-company 3com nic had been removed (again, registry -- which is how I knew 100% that's the machine I'm looking for.) That was the company computer for that desk, which is the only reason it was still there. Had he used one of his own machines, it could've simply disappeared without a trace. Moral of the story: I had a MAC, and it didn't tell me shit. The "who" was only discovered because his coworkers ratted him out :-) ("Who's been using X's computer?" They didn't know he was doing anything stupid with it.) And that only worked because it was in real time; had it been brought up a month later (nothing to trace) (and there not been a hub muddying the picture), any history logs would simply have ended with "I don't know. It was something at X's desk" [X having left the company many months ago. X's computer official off since it's builtin NIC, registered in company inventory, wasn't being used.]