I don't have time to work on this right now but I wanted to get this out there before I forgot and see what you all thought about it..
I read somewhere that the author said people were missing an obvious clue. In one of his other sculptures done after Kryptos (the one with russian) there is one english word - Medusa. I'm sure people have looked into this but given that Medusa could turn people to stone.. I'm thinking he may want to tell us that the petrified wood is important in some way. He's used it often. For one sculpture, he made it a point to say he had used petrified wood that had been burned before being petrified. I can't find anything that would indicate why burning first would make a difference or what kind of difference it would make. Perhaps none..but why "medusa"?