Ubuntu stores its password in /etc/shadow (have to be root to see it I think)
So if you use a liveCD, mount your filesystem , and then go edit the shadow file.
Here is a explanation of the shadow file http://www.cyberciti.biz/faq/underst...tcshadow-file/
I think if you leave the password part out of root. you would have a blank password.
the "!" represents the password for root.
I have never edited shadow file. Just saw someone do it one.