How to Downgrade Your iPhone 3G[S] from iOS 4 to iOS 3.1.3
http://lifehacker.com/5572003/how-to-downgrade-your-iphone-3g%5Bs%5D-from-ios-4-to-ios-313

lot of iPhone 3G owners—myself included—have discovered that upgrading to iOS 4 has slowed their devices to a crawl. If you decide the new features and shortcuts just aren't worth the glacial performance, here's how to downgrade your iDevice.