this link says that you can't do this generally, but that you can get the mac address from visited IPs in the arp cache.

Depending on your OS, you can get at that information in different ways. My linux box seems to store the IP/MAC info in /proc/net/arp, for instance. I have only one machine running here at the moment so it only shows my own address:

IP address       HW type     Flags       HW address            Mask   
+  Device
192.168.2.1      0x1         0x2         00:13:F7:27:FB:8A     *      
+  eth0
[download]

I suppose that you could ping all the IP addresses in your local subnet and then check the arp cache for matching MAC addresses.