I think its case by case. Some find it faster after the upgrade. I suspect its cause by some data or apps during the upgrade. Some find it running alright after doing a clean install. Or maybe its specific to certain batches of iPhone 3G hardware we will never know. Apple is rumored to release 4.0.1 pretty soon, so hopefully it will solve this slow issue on 3G hardware. iOS 3.x code base is Leopard base. iOS 4 is Snow Leopard. So by right iOS 4 should run faster if not smoother on same hardware base on Leopard/Snow Leopard upgrade experience.

Lifehacker has a guide: http://lifehacker.com/5572003/how-to-downgrade-your-iphone-3...