I'm going mainly by what I've heard from developer press releases, news stories, and android developer forums.
Here's a few that a quick google search yielded...
http://www.develop-online.net/news/4...80-piracy-rate
http://www.wired.co.uk/news/archive/...et-game-piracy
http://www.develop-online.net/news/3...an-iOS-edition