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On the inside, the iPhone 4 -- $199 for a 16 GB version, $299 for a 32 GB version -- offers a few other departures from the past that emerged during this morning's commute. (Apple's PR department loaned two 32 GB models for this review.)

The most welcome such change -- one also available to owners of the iPhone 3GS who install the iOS 4 operating-system update that Apple shipped Monday -- is multitasking. Or something like it. Although the iPhone 4 doesn't keep third-party applications running side by side, it allows updated versions of them to hand off some tasks -- for instance, music playback or tracking your location -- to the operating system before suspending their activity until you switch back to them.

On the iPhone 4, though, this seems to happen so seamlessly that you can't tell the phone is really performing "fast app switching." Tap the home button twice, and whatever you had on the iPhone's screen slides up to reveal a drawer showing any "open" applications. Slide a finder left or right to switch among them; a set of controls for the iPhone's iPod application await at the left of this list.

Apple talks with considerable pride about the iPhone 4's ultra-sharp, 960-by-640-pixel screen. But this 3.5-inch "Retina Display" -- named because, Apple says, the human eye can't even distinguish that level of detail from a foot away -- doesn't always look dramatically superior when you're looking at the iPhone's home screen. Only when you view a picture, zoom in on a map or open a book in Apple's iBooks program do the benefits of those extra pixels become obvious -- you can't see any jagged edges in images or text.

The iPhone 4's cameras represent a major advance over the hardware on earlier models. The back camera offers 5 megapixels of resolution and (finally) a flash, plus the ability to shoot video in 720p high definition. The front-facing camera -- intended both for self-portraits and for video chats using Apple's FaceTime software -- provides far less resolution, at just 640 by 480 pixels.

I'd been meaning to try FaceTime, which allows free video calls as long as you're on a WiFi wireless network connecting you to the Internet. But my attempt to launch a FaceTime chat with a co-worker using the second iPhone 4 failed without explanation: We could only see ourselves on our respective devices' displays, instead of being able to see each other. My guess, based on reports by other users: The Post network's firewall blocks FaceTime communication.

What else would you like to know about the iPhone 4 as I continue to work on this review? Post your questions in the comments. Or, if you were among the masses lined up outside Apple's stores or waiting for a delivery at home today, tell me what you think of your own iPhone 4. We've also set up a discussion forum for anybody running into unwanted issues with their new devices.

(And if you use a different sort of smartphone and want to point out how your gadget offered all of these features before Apple got around to adding them to the iPhone, you're welcome to do that as well.)