I've never had to reconfigure any of the WRT54GL/GS routers around here once they're set up. But mine all run the add-on "Tomato" firmware, rather than the stock Linksys stuff. That could have a lot to do with it.

Kind of odd that the MAC address changed -- it should NOT change, ever. Unless you change it on purpose (requires advanced knowledge, usually).

btw. The MAC address is simply the 48-bit unique network serial number of the WAN ethernet port on the router .. unique to your router, and no other piece of networking gear on planet Earth has that same number (in theory, at least).

Cheers