The tl;dr conclusion: nearly 99% of Instapaper users have iOS 4.0 or later, and nearly 95% have an iPhone 3GS-class processor or better. This jibes with what we see at Able Pear. We're planning to only support iOS 4.0 and later for all future app releases.
This is one of the things that Apple really does right with iOS. iPhone, iPad and iPod touch users upgrade their devices pretty rapidly; with iOS 5 allowing you to upgrade the OS directly on the device without connecting to iTunes, this will become that much easier and adoption of the latest OS versions will only improve in the future. As an app developer, there are pluses and minuses to Apple's tight control, but as an iOS user, this is a very solid plus -- upgrades are easy and developers rapidly support the latest iOS goodies rather than target some very old lowest common denominator OS version.
Check out the complete breakdown in