As a developer, what if I don't support iOS 3.1? I am currently developing an iOS application. I'm considering not supporting anything less than iOS 3.1. I was wondering, can I expect a huge backlash from customers that buy this application and try to use it on devices that haven't been updated in years?
You can find Marco Arment's stats on iOS version he collected from his Instapaper app (as CanuckSkier say in the comment, Instapaper is generally used by tech-savvy people, so "real"-people usage might be slightly less).

Here's a graph: 



As you can see, at least 98% of users are already on iOS 4.0 and 96% on 4.2. So I guess you can safely stop supporting iOS 3.x.