Using some form of scanning receiver and analyser? ... I'm only curious as I can't see how anyone could otherwise obtain the MAC address of my laptop?
All you need is another wireless card, in what is called 'promiscuous mode'. In this mode it listens to all the traffic on a wireless network, rather than discarding all traffic destined elsewhere.
Every time your laptop broadcasts a network packet, it includes it's MAC address in the packet. And as it's wireless, it's broadcast globally, rather than down a cable. I just need to sit in range of your wireless network, capture a packet from your laptop, and inspect it to find your MAC address. It's very simple, which is why a MAC-address ACL isn't security. It's just a simple barrier, and anybody with a desire to do so can bypass it. As I said earlier, it's unlikely that anyone will care enough to do so, but it's worth understanding the level of protection you have.
(edit - I answered 'what does listen in mean' with 'listen in', d'oh
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As a very rough analogy, think of an open WAP as leaving your front door open. The MAC-ACL is like shutting it, but I only need to watch you open it once to know how to work the handle. WEP locks it with a lock that I can pick, but it requires significant effort to actually do it. Of course, regardless of whether your door is open or locked, it's illegal for me to enter, but...