This extremely cool skeleton tattoo aside, I have to ask exactly what's the point of having a tattoo that you can't normally see? It's like, make up your minds, people! Either you want some art on your skin or you don't, yeah?
I have no tattoos, mostly because someone once told me that before you get one, you should be able to like the design for at least a year. Call me too variable, call me a gen-X short attention spanner, whatever - I've never been able to really love a design for more than a year. It always seems dated. The one tattoo that I have thought is consistantly cool for the last several years is daemon9's tattoo of a microchip die that he has between his shoulder blades. (I'd link a pic, but I can't find one any more...)