There are many reasons to disable ipv6 on Ubuntu (If you don't really need it). Among others:

• Some users report a noticeable improvement in network performance
• It's a workaround for a serious Tomcat6 bug

After a short Google session I found many (slightly different) 'solutions'. Nothing worked though. Eventually, I came across a solution that did the trick:

$ vi /etc/modprobe.d/blacklist

append the following line:

blacklist ipv6

save the file and restart your machine!

You can verify that ipv6 is actually disabled using the following command:

$ ip a | grep inet6

If you see not output this little exercise was successful. Go make yourself a cup of coffee!

BTW, I tried this on Ubuntu 8.10.