Localytics has a blog post that marks the first 2 weeks of adoption, but no real statistics have been released from Apple. I can tell you from experience that in one of my applications, most users on 4.1 (iPhone) and 3.1.3 (iPod) have only nominal representations of previous versions.
Adding any analytics to your application will always answer this question better than published statistics, since different user databases update iOS differently (hell, I'm still running 4.0!). Localtyics and Flurry have free accounts that let you track usage.