I preformed the usual upgrade to 12.04, and ended up loosing my Internet accessibility. Turns out you can corrupt the interfaces file in the etc/network directory.
To fix this, you need to make sure that the following content is present in "interfaces" file:
iface eth0 inet dhcp
iface lo inet loopback
I found that the first two lines had been removed, and as a result, I could no longer connect to the Internet. I changed the file permissions using the following:
chmod 777 interfaces