I made a thread about this a while back.
My opinion is that the only reason CD is more of a mystery than SMiLE is because after smile was shelved, the beach boys/brian wilson kept on releasing music, and a few of the tracks slated for smile showed up on the other albums.
Whereas guns n' roses haven't released anything since 1993 other than the intriguing 'Oh My God'. And it doesn't need to be said that there has been many frustrated fans wanting for any sort of music at all.
But if the chatter is right, we won't have to wait much longer...