I am running into the same exact problem. I have my valid MAC address however my IP is bogus (a 169). I have tried using both eth ports, disabling one at a time, and putting a manual address in and nothing has worked for me.
OK. Try this:
Disable in windows(and probably any other OS) the controller you will use for Mac OS X.
Convince your router if you have one to release all DHCP addresses, that is to clear it's DHCP allocation table. Add two entries in the manual DHCP allocation table for your two nvidia eth controllers. You could try a ipconfig /release as admin in windows to release assigned addresses. If this doesn't work reset your router.
After you are done with all this, unplug your computer for a while and boot in Mac OS X. After it has booted, plug in the cable in thge controller you want to use for Mac OS X(for me it was the first one because the second controller never detected the cable as plugged in - e.g. the controller with the lowest MAC address).
If you ask me, the forcedeth thing is too much of a hustle, I am thinking of plugging in my old Realtek controller any day now. Or even buying a new one. I just don't like packing things around my videocard and my PICe slots. I am also thinking of doing SLI in the future and things might get really crowded in there.
There's probably some bug in the IP broadcast code in conjunction of an already assigned DHCP address.
It's really weird.
Hope this helps.