I just confirmed this on my Mint install as I had never tried it because it is silly to log in as root or even su to root because sudo can accomplish everything that needs done in 99% of circumstances.
I was able to log in to a root GUI session in Mint 7 by using username root and my user password. I loged back out of that session and logged back into my user session and was also able to su to root in the terminal by also using the user password.
I find this to be dangerous, especially for those who use SSH . I bet I can hack into at least 5 user sessions of members in this forum by simply guessing their user names and passwords. In fact, just by looking for screenshots from members showing their terminal would give the user name. Most people create user passwords consisting of their name with a number, usually 1, or if they are a bit more creative their spouses name or their pet. I find this aspect of sudo to be a weak link and connecting a full root session to the user password is nothing short of stupid!!!
Be warned! Create a strong password!