Documentation for setting up native IPv6 on Redhat based distributions was a bit lacking so Ive included the steps for adding an IPv6 address to eth0 below. Ive also added details on how to add it to Debian based distributions.
Read the rest for yourself
In Debain and Ubuntu there is only one file to be editted, /etc/network/interfaces. You add the following:
iface eth0 inet6 static
One the file has been saved, run /etc/init.d/networking restart.