I don't think the MAC addresses are displayed in the standard debug.log (but you could check - viewtopic.php?f=6&t=3507&p=32731#p32731
), and although it might be in the trace.log, that would require enabling the trace.log and reconnecting your PS3, which obviously doesn't help you.
I think you're out of luck, at least as far as PMS getting you your MAC address. Sorry
Depending on your router and how long it's been, you may be able to look at your router's connection logs and get the MAC address from there.
You may also be able to look at your software firewall history, find the first inbound/outbound connection that you allowed from/to the PS3 and potentially get the MAC address from that.
I can't think of anything else at the moment.