Ubuntu maps the network card via the MAC address.  To change a card or boot a drive in a different system, do the following:

etc/udev/rules.d/70-persistent-net.rules contains the MAC address to eth device mappings. Delete the lines like below, noting the module name on the "# PCI device" line:

1. PCI device xxxxxxxxxxx (module)

SUBSYSTEM=="net", DRIVERS=="?*", ATTRS{address}=="xxxxxxx", NAME="eth0"

This removes the MAC to eth device mapping info. Now we need to restart udev to allow the change to take effect:

/etc/init.d/udev restart

Next step is to "bounce" the kernel module for the ethernet device. Use the module name from the 70-persistent-net.rules file noted above:

modprobe -r module
modprobe module

Check the eth? address now in the /etc/networks/interfaces to ensure it matches the new files in the udev file. 

/etc/init.d/networking restart

 "ifconfig" should now show the eth0 interface as up and running.