I had a look and it seems sis900 is compiled in directly rather than as a module, which is why I couldn't find it, and I assumed it wasn't present at all as it wasn't working, and it did exist as a module in the 2.2 install. Thanks for the sanity check.
It was working in RedHat 7.3 (migrating away from RedHat), it works in Knoppix 3.2, and in XP. I checked dmesg and I'm seeing:
sis900.c: v1.08.02 11/30/2001
eth0: Error EERPOM read ffff
(no, it's actually spelling it EERPOM
). Not sure why it fails with Woody when it's working with everything else; I'll look into it further from this perspective.