We could go on at length, but this is supposed to be a brief overview. Our iOS 4 review has more details, and we'll have plenty to say about less heralded features in the next few days.

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.

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.