Done by Craig Brock at Black Heart Tattoos in Stockbridge, GA. It is an ambigram of the word "Atheist" meaning it looks the same right side up and upside down.
Do you mind if I ask where you found this design? I created it in 2008 but can't remember where I posted it. I have found quite a few people who have this tattoo to my surprise. I don't mind, but am curious.