Actually, we are going to test a good anti-piracy method soon ourselves. I'll probably post results on how it goes.

It comes down to this: there is no 100% sure way of stopping piracy, unless you have an On-Live style game. Even then server hacking could be done.

Look at it this way, if you can write a code to encrypt a file. Someone can decrypt it. Even if someone has to buy the game to release it as a torrent or otherwise, someone will do it. The best solution is to take some anti-piracy measures without filling your software full of a bunch of stuff that could bug the hell out of anyone who buys it. Simple cd-keys for unlocking a game are fine, don't over-complicate it because even after spending a bunch of money on a great program, someone will find a way to break it.