Good question.
It's been covered pretty well before, too.
Generally, most places forbid tattoos to keep out yakuza. Rules are rules, so don't expect different treatment if you're a foreigner. Of course there are exceptions and stories of people getting in with them. Personally, I've seen people asked to leave because of them. I also know someone who just puts duct tape over his and goes in with out any hassles. Not sure if the place restricts them? Most places will post a sign at the gate saying if they do or don't. And if you can't read Japanese, I'm sure they'll understand you if you just show them!
:sorry: