I have been curious about this myself. In my heart, my conscience tells me it is a sin to tattoo or pierce my body.
I have asked others about this. My priest apparently told my fiancée (who is being tempted into getting tattoos) that there is no canonical restrictions on laymen. Only clergy. Likewise, my godfather explained to me that there was no canonical or official stance from the Church as a whole, thus making it an issue of one's conscience.
It does not seem right to me. As I have explained to my fiancée, it is like spray-painting graffiti on a temple. And while the cleanliness of the inside of the "cup" should be prioritized first, it does not seem a light thing to disregard the outside as well.
Any thoughts?