SAN FRANCISCO (AP) — Apple CEO Steve Jobs briefly emerged from his medical leave and walked on stage to a standing ovation Wednesday to unveil the second generation of the popular iPad. It comes with two cameras and will go on sale March 11 in the U.S.

Jobs looked frail as he appeared in his signature black mock turtleneck, blue jeans and wire-rimmed glasses.

"We've been working on this product for a while, and I just didn't want to miss today," Jobs told an audience that included bloggers and Apple enthusiasts. "Thank you for having me."

The next-generation tablet computer is faster than the original iPad's. As expected, it comes with two cameras for taking photos and video chatting. The battery life will be the same as the original — about 10 hours of usage or a month on standby.

The iPad 2 is also thinner — 8.8 millimeters, or about a third of an inch, instead of the current 13.4 millimeters.

"The new iPad 2 is actually thinner than your iPhone 4," Jobs said.

The original iPad, which went on sale last April, was more popular than analysts anticipated. Apple sold 15 million in nine months.

The iPad was initially used for checking e-mail, surfing the Web and watching online video. But as the number of software applications — or "apps" — designed just for iPad grew, the tablet made itself at home in offices, shops, restaurants and countless other settings.

The rush for iPads sparked dozens of copycat touch-screen devices, but so far, none has broken into the mainstream consciousness the way the iPad has. In February, Motorola Mobility Inc.'s Xoom, the most promising challenger so far, went on sale. It runs a new version of Google Inc.'s Android software that was designed for tablets, not smart phones.

The new iPad will make it even harder for rivals to compete.

"Overall, the big message today is that Apple is offering a version 2 device while everyone else is still attempting to ship their first version 1 devices," said Yankee Group analyst Carl Howe.

He said the iPad 2's improvements are modest over the first one, but it will nonetheless stand out because there are more apps available.

Sarah Rotman Eps, a Forrester Research analyst, said iPads should make up at least 20 million of the 24.1 million tablet computers she expects people in the U.S. to buy this year.

Tablet computers existed long before the iPad, but it took Apple to build a device that made sense to consumers. Apple simplified the software, packed it in sleek, shiny hardware and sold it to a generation of gadget lovers who, most likely, already have a smart phone and a laptop that serve most of the same functions.

The new iPads will cost the same as the originals — $499 to $829, depending on storage space and whether or not they can connect to the Internet over a cellular network. Apple said there will be black and white versions, despite its problems getting the promised white iPhone 4 models to market. The first iPad came only in black. In the U.S., the iPad 2 will work on AT&T Inc. and Verizon Wireless.

A reporter who used a white iPad 2 immediately after the announcement found it noticeably thinner and more curvy. YouTube video loaded quickly using AT&T's data service, and "Toy Story 3" played smoothly. Given its size, the iPad 2 appeared impractical for taking lots of photos, but both cameras will help with video chats — the front one to show the caller, and the back one to show what the caller is seeing.

Jobs also introduced a new accessory for the iPad that will let people connect the tablet to high-definition televisions, so they can watch videos up to 1080p in resolution on the bigger screen. The $39 part plugs into the iPad's charging port and connects to an HDMI cable.

After its March 11 U.S. launch, the iPad 2 goes on sale March 25 in at least 26 other markets, including Mexico, New Zealand, Spain and other European countries.

Apple also introduced updates to the software that runs on the iPad, iPhone and iPod Touch devices. The company said the update would work on GSM-type iPhone 3GS and iPhone 4 models; Verizon Wireless' version uses a different technology.

The new system, iOS 4.3, includes support for FaceTime, Apple's video-chat program. The company said people can now hold conversations between iPads, iPhones and Mac computers.

The update turns iPhones and iPads with 3G cellular connections into personal Wi-Fi hotspots, so you can share the connection with computers or other devices — if your wireless carrier allows it. Many charge additional fees for this service.

The improved software also makes Apple's Safari Web browser run faster.

Apple also announced new software designed for the iPad, including a $4.99 version of iMovie for video editing and a $4.99 version of GarageBand, its music recording and editing software. GarageBand includes instruments that can be played by touching the iPad 2's screen, and it can even sense whether you're tapping quietly or banging on the "keys." People can start a project on their Mac, then work on it later on the iPad 2.

Jobs gave an update on the company's iBook business, saying people downloaded more than 100 million books since the e-book business launched last year. He also said Random House became the last major publisher to agree to sell its titles in the iBookstore.

Jobs announced in January that he would take a third leave of absence to focus on his health. In the last decade, Jobs, 56, has survived a rare but curable form of pancreatic cancer and undergone a liver transplant.

Chief Operating Officer Tim Cook has been running day-to-day operations.

In 2009, Jobs said in advance that he would take a six-month medical leave; this time, the company did not specify when he would return. The last time Jobs went on medical leave, marketing chief Phil Schiller was the main presenter at product launch events.

Shares of Apple, which is based in Cupertino, Calif., rose $1.66, or 0.5 percent, to $350.97 in afternoon trading Wednesday.

Image: AP Photo/Jeff Chiu