OVAs are a handy way to bundle up virtual machines with all their configuration in a way that is portable between different virtualisation environments – you can export from VirtualBox and import into vmWare, for example. You can even bundle up multiple VMs, like an app server and its database, together.
I have a little virtual development server I use for Network Weathermap development, which I made an OVA of, to avoid having to set up Cacti several times on different PCs. Trouble is, when I used my OVA, the network didn’t work…
Once I’d figured this out, it was pretty logical, but when Ubuntu detects a new ethernet interface, it adds a line to /etc/udev/rules.d/70-persistent-net.rules which defines that interface as eth0, eth1 or whatever.
When you create an OVA from a running system, it has already done this. When you import the OVA and choose ‘Reset MACs’ (which you probably should, rather than have two VMs with the same MAC address), then Ubuntu detects this ‘new’ NIC. The result is that your config for eth0 is ignored, because eth0 has gone away, and this mysterious new NIC has appeared with a new MAC address.
Delete the ethernet-related lines from /etc/udev/rules.d/70-persistent-net.rules and reboot, to be back in business.
Update: Also true for CentOS 6 (and presumably RHEL 6 and Scientific Linux too).