Apple just revealed the iPhone for 2010: The iPhone 4. It's the design we've been expecting, but there are a number of surprising and amazing features that we're only learning about now, one of which is a human-retina-resolution screen.

Gizmodo did indeed have the hardware for this year's iPhone when they made their controversial leak postings, and in that early look at the hardware (even though it wasn't properly working) there were suspicions that the iPhone 4 had a super-high-res display. Steve Jobs just introduced it; the Retina Display.

While it's in many ways a standard LCD, with 800:1 contrast ratio and millions of color possibilities, the screen leverages the same IPS (in-plane-switching) system that the iPad does to pack four times as many pixels onto it than the previous iPhones do. That gives it a 960 by 640 pixel resolution on a 3.5-inch screen. This makes it easy for existing apps to work on the new phone--they simply have to double pixels in both directions on screen, and they'll appear exactly as they do already.

But that resolution, when used by new apps, will equal a pixels per inch count of 326. As Steve Jobs noted during the presentation, the human retina has a detection limit of around 300 pixels, making the screen look incredibly smooth. Jobs even suggested the look of the screen for text approached the quality you could expect from a printed page.

There are other significant changes in other parts of the iPhone hardware too. Here's a quick summary.

• Antennas: For those of you, like me, who were wondering where the heck these were in all those leaked tear-downs... The iPhone 4's metal case is the antenna, which explains those electrically insulating black strips around the side, separating the Wi-Fi and 3G antennas. The metal is stainless steel.
• It's 9.3 mm thin, 24% thinner than the iPhone's existing version, the 3GS.
• It's got a bigger battery, and better power management so there's about 40% more battery life. Apple's saying 300 hours of standby, 10 hours of Wi-Fi Web browsing and 7 hours 3G calling.
• That mysterious back is glass too, scratch resistant and drop-resistant like the front.
• The processor inside is the iPad's Apple own-brand A4 ARM unit, which means the iPhone 4 easily outpowers its older version.
• The motion sensors are joined by a gyroscope, for more accurate positional sensing. There's a suite of API hooks to support this new power, and it'll transform applications like GPS, star maps, and Augmented Reality.
• The front-facing camera is there, for much-anticipated video calls and VoIP video.
• The rear camera is a 5-megapixel unit, with the emphasis on good imaging, not the pointless race to more megapixels. It's a rear-illuminated unit, which improves its quality, and has five times digital zoom. There's an LED flash. It records 720p video at 30fps, making it really an HD video camera.
• To back up the new video powers, Apple's releasing iMovie for iPhone, enabling complex video editing on the device itself. This is actually a big deal, representing the first new i-prefixed software that's available for the Mac moving to the iPhone since iWork had an iPad make-over. iMovie does transitions, edits, and music inserts, and is a $4.99 app on the App Store. It'll transform the iPhone into a serious video uploading resource. Bye bye, Flip?
• The operating system for the device is getting a rename from iPhone OS 4.0 to iOS4. This makes sense given that the "iPhone" OS is also used on the iPad. But does it also suggest that Apple has bigger plans for iOS? As in those juicy rumors about the refreshed Apple TV?

Update:

Steve Jobs did a long-missed "one more thing" to mention the iPhone 4's video-chatting powers. Using a special app dubbed FaceTime, you can chat iPhone 4 to iPhone 4 anywhere there's Wi-Fi ("Wi-Fi only in 2010. We're working with the cellular providers to get things ready" ... a shame, since 3G video calling's been available for many years elsewhere in the world, and the U.S. more recently). The system can use either camera, and they'll be making the app an open standard, to try to spread its compatibility.

To keep up with this news follow me, Kit Eaton, on Twitter. That QR code on the left will take your smartphone to my Twitter feed too.