I was just searching the forums when I came across a post from mister Klaus Knopper himself. I haven't had a chance to try it yet but hopefully this will help alot of people who are using DHCP to get their internet back up after a hdd install.

The harddisk-installed system is a "normal" Debian. You have to add the
lines

auto eth0
iface eth0 inet dynamic dhcp
I think this should be in the FAQ (if it was I missed it)