I agree, it sounds like a lot of hot air to me. Most people, when they receive these threats, will shit half a brick and comply. But if you run it by someone who's knowledgeable of such matters they'll probably laugh. For instance:

http://thepiratebay.org/legal - I think MetalTabs is a hell of a lot more innocuous than a P2P website.

Ask a solicitor for advice. It's usually free, and if not it's pretty cheap just for a cursory assessment of the situation. If you have any legal fees, put a donation drive on the front page or something. You've got my cash, that's for sure.