I've got this problem two times already, first during an VirtualBox upgrade, the second time during a motherboard change. Since i've lost time the two times, i'm now writing down the solution.

Udev has a cache of network interface mac addresses, in order for you to always get the same interface for one device. In order to solve the disappearing eth0 problem, you need to clear the Udev mac cache.

There is 2 ways to do this:

1. Manually edit the file /etc/udev/rules.d/z25_persistent-net.rules, and remove the faulty lines.
2. Brutally trash this same file:
   

   /bin/echo "" > /etc/udev/rules.d/z25_persistent-net.rules

After doing theses changes, you need to restart your computer in order to take account of the changes.

Thanks

• Thanks to gnuser for his work on the ticket VirtualBox n°660 : Cloning Debian Etch guest breaks networking.