I have washed the pans in the dishwasher and the ink hasn't come off yet (I heat set them in the oven). Someone told me though that when you scrub them with like a brillo or something coarse you can scratch the ink off!
Yes I did stamp on the outside's of the pan! Just to make sure it didn't get into the food! I thought I read somewhere once that the ink was safe for consumption though (someone was stamping on cakes)...