World of Goo designer Ron Carmel has claimed in this RPS thread that World of Goo has a 90 percent piracy rate.
Carmel claims he's seen torrent sites with "500 seeders and 300 leechers" and has received emails from people bought the game after downloading it, although that number makes upa "very small percentage."
Luckily, he said, "We're doing ok, though. We're getting good sales through WiiWare, Steam, and our website. Not going bankrupt just yet!"
Phew. Thanks Joystiq.