When iOS 4 is released June 21, it is available for free download for all iPhone and iPod touch except first generation devices. But you will need iPhone 4, iPhone 3GS and third-generation iPod touch (32GB and 64GB models from late 2009) in order to enjoy full iOS 4 features such as multitasking. The following is what we know of are missing features in older devices as there are no official list yet from Apple:
* Custom wallpaper on home screen
* Bluetooth keyboard support
* Orientation lock as this feature requires background processing
Please let us know if there is any not in the list.
Updated Sep 8, 2010: Apple [confirmed](http://www.apple.com/game-center/) that Game Center which is part of iOS 4.1 update, is not available for iPhone 3G. Game Center however is compatible with second generation iPod touch which shares similar hardware with iPhone 3G. Both iPhone 3G and 2nd generation iPod touch were introduced in 2008, but 2nd generation iPod touch sports a faster processor (532MHz vs 412MHz) than the iPhone 3G.
Updated Sep 8, 2010: It is confirmed that HDR photo, part of new feature in iOS 4.1, is iPhone 4 only. Not even iPhone 3GS has this new feature.
Updated Sep 10, 2010: iPhone 3G and 2nd-gen iPod touch loses [over-the-air Notes syncing feature](http://support.apple.com/kb/HT4191) with iOS 4.1 update.
Updated Dec 02, 2010: AirPlay and AirPrint, new features in iOS 4.2, are not available for iPhone 3G and 2nd-gen iPod touch.