Personal thought: I think the tattoo looks a bit like that W.H.A.T. wind turner thingy on their house (have no idea what it's called, lack of knowledge in english)
You can see the arrow and the W sign.
I think that it's a map or something.
I third what Waddles said.
That little blobby looking thing that's on the highest part would be the part of the '?' that bows out, and there's a little arrow thing with a 'w' right above it.
Or maybe it's something we've never seen before, but with the images that we've seen I would think that that theory makes the most sense.
It also seems to change. If you look at the part that sticks out from under his shirt in the first two screenshots, it's the same arrow (Anyone know what episodes those are from?) but in the third, it's an exclamation mark.
I also think that the wind-tuner theory is very possible.
It kinda reminds me of a map compass or a boat anchor... maybe he was in the Navy once.
It could possibly be a combination of the two, also; even if the little part we can see is that wind-thing (because I can't remember the term right now), it could possibly be pointing to something on a map or even be a landmark on the map itself. We only see that little part, and there's the whole of his back so we don't know how big the tattoo itself is.