Yeah, that's normal. The ethernet ports have a different MAC address from the wireless port. I've never bothered to check if the MAC address on the serial number tag is that of the ethernet ports, or the wireless port.
Not a problem. If you're not sure which to use, just:
Connect to the WRT54GS via wireless (without the WRE54G). Make sure your computer is NOT also connected via a wired ethernet connection.
Open a DOS window and run: Start -> Run -> cmd arp -a
You should get a list of MAC address and IP address pairs. The one next to 192.168.1.1 is your router's MAC address. For example.
arp -a Interface: 192.168.1.11 on Interface 0x1000003 Internet Address Physical Address Type 192.168.1.1 00-0c-41-71-36-30 dynamic