I realize this is considered solved, but would like to mention that an alternative to enabling the root account exists and should be considered. First, this (Mint8) is the first time that I have not enabled the root account and have not found a good reason to do so at this point. I'm a bit stubborn and have always thought it best to have root enabled. But I may be changing my mind. But I digress --
From Terminal $:
The above will allow you to become root until you 'exit'. Furthermore, this will use the root environment, which I prefer and consider "truly root". An alternative would be to not use the dash in which case you will have root power but using the environment variables for the user account from which this was launched.
Also, the only time I become root in Mint is to do a relatively large amount of administrative work for which prepending 'sudo' to the front of every command is toilsome and I sometimes forget to do so. But for the most part, using sudo is the way to go while in Mint. (The jury in my head is still out on what should be my practice whilst in Debian.)
BTW, I thought I had a graphics (or Evince) problem recently so I did a recovery boot and eventually felt compelled to drop to a root console. Not having enabled root -- that is, I never assigned root a passwd -- I was delighted to find that the admin password was considered to be the root password there. Upon returning to a regular session, I found that the admin passwd also passes as the root passwd so all one really needs to type is 'su -' to become root. But again I'm stubborn. ;-)