I tried everything I could think of, and of course even with the NIC enabled in the BIOS nothing was working. The light would flash when you plugin the cable for a second, but that's all.
Due to another issue I'm about to post about (server is not compatible with 1TB/1000Gig Hard Drives), I updated the BIOS. I didn't even know the 100mbit NICs were not working until I decided I should test each NIC one by one.
I noticed that only 1 server out of the stack I have had a working 100mbit NIC. I was thinking what are the chances of this and how could all of the 100mbit NICs be bad? Then I realized 1 server actually worked with the 100mbit NIC and it turned out that was the one I got the latest BIOS update for.
So if you have a Tyan 2735-8M motherboard and the 100mbit NIC is not working, update your BIOS (upgrades available from the Tyan website) and you'll be good to go.
I guess this probably goes for other Tyan motherboards and even motherboards from mother manufacturers. It is a bit ridiculous though that in the original firmware they made such an oversight that the 100mbit NIC doesn't even work. This must have caused many sysadmins/IT techs headaches, headscratching and frustration.