As expected, Steve Jobs announced the next-generation iPhone at Apple’s World Wide Developers Conference in San Francisco, and it looks like another breakthrough. The iPhone 4 offers many new features that will appeal mainly to affluent consumers with innovative camera and display options.

The most visible feature is the new higher-resolution Retina display. While it’s still 3.5 in. diagonal, it offers 960 by 640 pixels for 326 pixels per inch. That’s beyond the limits of the human eye’s ability to discern individual pixels. This higher resolution is striking, and it’s a real benefit for users who want to watch video and play games.

The iPhone 4 is the same size as the previous models, but it’s a little more squared off (see the figure). Its front and back both are made of a special super-hard, scratch-resistant glass held together by a thin stainless-steel band. At 9.3 mm, the iPhone 4 is thinner than previous editions, and Apple calls it the thinnest smart phone on the market.

Also, the iPhone 4 offers a 5-Mpixel camera with an LED flash, HD video recording capability, and 5x zoom. It even includes a second camera that faces users so they can have face-to-face video phone calls thanks to Apple’s FaceTime feature. With just a tap, users can switch from the front camera to the back camera so the person they are speaking with can see what they are looking at too.

The phone is still a 3G device, though, using HSPA on AT&T’s wireless network. Its Wi-Fi 802.11b/g is a given. Apple’s own ARM-based A4 processor runs the electronics. A three-axis gyro joins the already present accelerometer for six-axis motion detection, making gaming and other clever apps better than ever.

Users won’t have to worry about the battery either. Apple says that the iPhone 4 offers 40% longer talk time for a total of seven hours, 10 hours of Web browsing using Wi-Fi, six hours of 3G data usage, 10 hours of video playback, and 40 hours of iTunes playback. That’s a great achievement, considering all the new features.

The iPhone 4 comes with a new operating system too, designated iOS 4. Some of its 100 new features include multitasking, folders, enhanced mail, enterprise support, and the iAd mobile advertising platform. There are more than 1500 new application programming interfaces (APIs) for apps developers as well.

While the iPhone 4 is sure to be a big hit with smart-phone power users, Apple is still third in the smart-phone category, with Nokia in first and RIM in second. Rival phones like the Motorola Droid and HTC EVO offer about the same performance and features. But despite the competition, Apple’s marketing magic and image will keep the iPhone number one in the consumer’s mind.

The iPhone 4 will be available June 24 in Apple’s and AT&T’s retail stores and online. The 16-Gbyte memory model costs $199 and the 32-Gbyte model costs $299 with, of course, a new two-year contract.

Apple