Like Amanda, I don't think it's much of a problem to cover them up, otherwise actors wouldn't get them. I think make-up is checked (and fixed if necessary) between takes anyway. I've never noticed any of Heath's tats (or of other actors) in other movies.
Well, my thought was that it must be
somewhat of a problem to cover them up, or we wouldn't see them in BBM, either. I know it can be done, but it must not be foolproof. I remember years ago reading a quote from Cher, who had just gotten some big tattoo, saying she knew it meant she would never get another movie role again but that's OK, she really wanted the tat anyway. Now obviously it's not THAT problematic, but you'd think it might be more trouble or risk than it's worth. Especially for tats in really conspicuous areas, like the wrist.
As for piercings, I think that after they've been there for a while they do not grow back very easily.
Then again, here we are talking about a medium where they can make people look 50 years older or like monsters or Dr. Seuss characters or give them serious wounds or whatever ... so I guess covering up a tattoo wouldn't be an insurmountable problem!

Anyway, sorry to derail the thread, but I've always wondered that. Just as I wonder when I see cashiers with long, carefully polished fingernails.