An updated official support page: "Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings."

Photo: Blake Patterson (cc)

http://feeds.boingboing.net/~r/boingboing/iBag/~3/EfasWhxv7RU/apple-admits-tats-are-a-proble.html