The iOS 4.1 upgrade seems a lot snappier on my iPhone 3G.
I have a sneaking suspicion that Apple knew that iOS 4.0.x's performance sucked on the 3G before they shipped it, though, but decided to ship it for 3G anyway. Who knows, some people inside Apple might have hoped that the slow performance of their old phone would help convince them that it was time to upgrade to the iPhone 4.
Sorry Apple, but your flaky "upgrade" only succeeded in pissing me off. As soon as Verizon has an Android phone as good as the Evo 4G, I'm defecting to the Android camp. Test your shit better before releasing it next time.