Comment 13 for bug 149869

After having upgraded from Kubuntu Feisty Fawn to the Gutsy beta Release and a few upgrades down the road, I had a very similar issue, and commenting out (i.e. adding a # to the start of the line) the lines starting with iface in my /etc/network/interfaces, seems to have solved it for me.
I think this is more or less the upgrading issue mentioned on http://www.ubuntu.com/getubuntu/releasenotes/710 under Network Manager.

Michael