This iOS support matrix (Autumn 2012 edition) posted by app developer Empirical Magic is definitely a useful resource for developers, the chart shows the number of devices running certain versions of iOS, and makes recommendations for dropping support in future updates.

An Australian iPhone user found out a hidden matrix code behind the rear plastic clamshell of his iphone while he was playing around with the night shot mode of his camcorder.
» Hidden matrix code on the iPhone 3G