A few things are gone, but we’d wager you won’t miss them. For devices that support multitasking, double-clicking the Home button no longer summons up iPhone controls, jumps to the Home screen, or brings up a list of Phone favorites—double-clicking is the domain of multitasking now. (The double-click of the Home button works the same as it did before on the multitasking-free iPhone 3G and second-generation iPod touch.) When you delete an app in iOS 4, you no longer get the option to give it a mouse rating before the app disappears into the mists of time. And, as noted in our article on changes to the Photos app, you no longer can copy an unlimited number of images: iOS 4 users can upload one photo at a time to MobileMe, select a maximum of five images to e-mail or copy, and text as many as nine images in one message.
What if I don’t like iOS 4? Can I downgrade to iOS 3?
Perhaps the best answer to this question is “kinda.” Apple offers no official downgrade path for iOS, largely in part because each upgrade makes significant changes to firmware—the low-level chips that power things such as the wireless radios for making phone calls and transmitting data.
However, the “jailbreak” community that creates tools for making unsupported modifications to iOS usually devises a way to perform such a downgrade. Like Macs, though, an iPhone can only be downgraded to the version of software that it shipped with. For example: an original iPhone 3G may be able to be downgraded to iOS 2.x, and an original iPhone 3GS might be able to be downgraded to iOS 3.x. But an iPhone 4 will only be able to run iOS 4. It is also worth reiterating that these jailbreaking tools are not supported in any way by Apple. In fact, the company reserves the right to void your device’s warranty if it discovers that you have used such tools, so tread carefully.
All things considered, it is rarely worth the effort to downgrade to a previous iOS version. If you are having troubles with a new version of iOS, your best bet is to seek official help from Apple, ask for community assistance in our Macworld forums, or simply try to make due until Apple inevitably releases a bug-fixing update.
[Updated 3:43 p.m. PT to add detail about missing iPod controls and orientation lock on 3GS and 2G iPod touch models.]