It's important IMO to consider how long an iOS device can use the latest firmware. Once you cross the 2-3 year barrier most people are looking to upgrade anyway so they don't consider it a big deal vs. not being able to upgrade a 6 week old phone.

EX: iOS 6 comes out this fall and is backward compatible all the way to 3GS phones which came out June 19, 2009, but not 3G which came out July 11, 2008 which they stopped supporting with iOS 4.2.1 on November 22, 2010.