According to data gathered by TorrentFreak earlier this month, StarCraft II is the most pirated game of 2010 - to date, anyways. StarCraft II: The Wings of Liberty has been downloaded more than 2.3 million times since the game released in July. According to TorrentFreak, the total number of data downloaded as it relates to StarCraft II is a staggering 15.77 Petabytes. This data is based on torrent downloads, and does not take into account downloads from other non-file-sharing sources.
Around 3 million legitimate copies of the game have been sold to date. Of course, those 2.3 million copies can't be used to play online, so that's something.