Thanks for the comment Sebastian – you were right I did lose sudo permissions with that command! I have updated the command in the post above to include the -a switch – many thanks.
I have posted my own troubleshooting steps here: http://pricklytech.wordpress.com/2011/10/16/ubuntu-linux-mint-user-is-not-in-the-sudoers-file-this-incident-will-be-reported/
Thanks again for your input – it is much appreciated!