I am just writing this in hopes that someone with the same problem in the future does not have to go through what I did and can find this info on the internet.
After upgrading the firmware on your new linksys wrt54g router, if you are macintosh user, your router will no longer connect to the internet via ethernet, like it used to BEFORE the firmware upgrade. It took me two routers, sending one back and getting another and then upgrading it again to figure out there were problems. The firmware upgrade ( sept. 2006) somehow makes it so that you cannot connect via the ethernet anymore.
The solution is to set up a static ip address. I did this through their tech support at linksys. This problem will continue until they release the next firmware upgrade. Who knows when that will be.
again, this is what happened to me:
1) plug in newly purchased router. works great.
2) now, time to set up your secure wireless network. the tech will tell you to upgrade your firmware at this time ( though he/she should really not be telling you to do this if you are a mac user or should tell you about the problems you'll have)
3) after the firmware upgrade, your mac ethernet connection will be screwed.
4) setup a static ip address and you are ok
5) wait for the next firmware upgrade untiil you can surf with a dynamic ip.