I did some more thinking about it, and then I found how to fix and recreate the problem.
Let me define a cold boot versus a full power off. A cold boot would be shutting the system down into "power off" mode, but the 5V power on the motherboard is still there. Pulling the
plug from the wall is a full power off.
So... from a full power off, the LiveCD boots fine and forcedeth 0.48 works great.
Then, I can reboot and load Win XP. But when I reboot from Win XP, forcedeth will not transmit any data. Reading on the kernel mailing list, it sounds like I should recompile the forcedeth driver with debugging enabled to see what's going on.
Once Win XP has hosed the network chip, even powering down and a cold boot isn't enough to get forcedeth to work. However, loading nvnet (the nVidia nforce module) will fix the problem. The only other way to fix the problem is to unplug the power supply or switch it off at the power supply.
Really OFF.
I don't know what the Win driver is doing, but I don't like it.