Information on the subject of iOS 4.3 device support is pretty thin on the ground at the moment. However, the lack of a 4.3 beta for iPhone 3G devices (and older iPod touch devices) leads many to believe this could be the end of OS upgrades for older devices.
Even if this assumption is incorrect right now, one day it won't be. There are hardware limits that will eventually stop Apple from supporting the oldest of devices with new iOS updates.
Given that iPhone application development debugging has the requirement of both iOS SDK and iOS version being the same, how would one continue development using iOS 4.3.x SDK releases while doing device performance testing on an iPhone 3G (or older device)? Are there ways of debugging over different iOS/SDK versions? Or do we have to simply drop development support for the older devices and tell those customers who don't upgrade their hardware: "tough"?