Wow I didn't notice that or don't remember thinking about it.. I would assume after this last episode of Lost "Something Nice Back Home" that he gets tatoo's as he hit milestones in his life.. Maybe the writting was the first.. Then he added to it.. Then we now see a new one on his back when he is off the island.. Kinda pic telling a story..

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Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.