IPhone fragmentation

I'm the (proud) owner of a first generation iPhone and I'm also pretty addicted to buying apps on the App Store, it's such an amazing experience.

Since iPhone 4 was released, this experience started to show some problems. I like to check what's being released, I check it daily, and when I find something interesting I just try it. The problem is that sometimes the app is not available for my iOS version, the latest for the first generation of the iPhone (3.1.3).

This problem is getting more frequent so I was wondering: is it worth to choose not to develop for the 3.1.3 version leaving all those 6 millions[1] of first generation iPhones out? What's really unique about iOS 4 SDK to make someone develop only with it in mind?

[1] http://en.wikipedia.org/wiki/File:IPhone_sales_per_quarter_simple.svg - I have no idea how many iPhones were sold to this day but it seems like the original iPhone is 10% of the total units sold.