One thing is very clear from Marco Arment’s latest device usage statistics for his Instapaper application. iPhone and iPad users prefer to have the latest operating system on their devices. No less than 87.8% of all devices are using OS 4.3 at least while the latest OS 5.0 has been installed on 45.1% of devices. iOS 4+ is already so ubiquitous, according to Marco, that it is now safe for developers to require it.
A word of caution, however. I suspect that users of Instapaper tend towards the geeky end of the spectrum and are therefore probably more likely to be early adopters of the latest software. Still, some impressive statistics. Read Marco’s full analysis here.