Comment They tried this in the UK... (Score 4, Informative) 197
And the courts weren't happy about it. The wording of the letter has to be extremely precise and boils down to:
We think you've been downloading porn, but we can't prove it. We'd like you to pay us some protection money, but legally we can't force you to without you agreeing to show us what's on your hard drive.
So, there's no onus on you to pay up whatsoever. Remember that IP addresses are still not considered solid evidence in most countries. To the extent that if your laptop gets stolen and you have an IP address and a GPS fix on the crook's address, the police often refuse to go round and batter the door down because it's they need more proof.