My G4 400 single processor is also afflicted by this 10.2.8 issue with SED (sudden ethernet death). The PPP light on my ADSL modem died almost instantly and after several hassle-ish calls to my provider etc (in difficult Japanese no less) I realized the problem was closer to home.
Have downloaded the older GMACEthernet kext file (bless you Andrew McPherson) and am on my way home to try it out. The Apple fixes posted on Macfixit are disapointing to say the least - the archive re-install method is pathetically slow and cumbersome.
Hopefully someone somewhere is taking note here. I just had to explain my issue to a Windows-centric colleague and was suddenly, accutely aware of how "Windowsy"and "Microsofty" the whole, sorry issue sounded.