Many iPhone owners have just updated their devices to the latest iOS 4 without even running a compatibility test to know if the features would work or not. This means that there are many features like the Home screen folders, Unified Inbox and threading for Mail, Internet tethering which would run on both iPhone 3G & 3GS devices but features like Multitasking, Bluetooth Keyboard Support, Home screen wallpaper & Data protection are not available for 3G devices. Using the following charts you can find out what all features would be devices for which devices.
Similar rules are applicable for the iPod Touch devices where a very less number of features are available to iPod Touch 2nd generation devices while the 3rd generation owners can enjoy all the new features. The first generation devices are completely unsupported which means that they either have to sell away their devices and buy the new iPhone4.