Recent research from 24/7 Wall Street calculates that $450 million has been lost through pirated versions of apps appearing on the IPhone or IPod touch. To put this in perspective this equates to about a year of gross revenue since the app store went live in July 2008.
This trend has accelerated as the ability to jailbreak / unlock these devices has become easy and does not require programming knowledge. Games developers, such as the excellently named Smells Like Donkey Inc, report piracy rates as high as 90% for the paid for games they have developed.
From an Apple perspective the core objective is the sale of handsets so there is little incentive invest resources in dealing with the Pirates. However - the content providers do need to become more active to avoid a generation of consumers expecting premium content to be free.