Comment 7 for bug 611226

I also had this issue. I saw a good number of questions but no answers so I am linking to my resolution in the forum.

http://newyork.ubuntuforums.org/showthread.php?p=9839614#post9839614

It may not be a bug but hostname -f is definitely picker in 10.04 than it was in 9.10. I duplicated the same config in both (via a script using the official cononical EC2 AMIs for each) and the simple setup works in 9.10 and not in 10.04. editing /etc/hosts and /etc/dhcp3/dhclient.conf gets me to a working solution in 10.04.