The company’s stores in major U.S. cities had sold out of the updated tablet device within hours of its introduction at 5 p.m. local time Friday, according to surveys by analysts. On Sunday, Apple’s online store was showing a shipping delay of three to four weeks for all of the versions of the iPad 2.

Analysts put sales of the new iPad in the range of 400,000 to 600,000 units during its first three days on the market, about the same range as the original model sold in its first week. Apple hasn’t disclosed any sales figures yet.

`IPad 2s are also being sold by mass-market retailers like Best Buy Inc., Target Corp. and Wal-Mart Stores Inc., as well as Apple’s wireless-carrier partners AT&T Inc. and Verizon Wireless, a unit of Vodafone Group PLC and Verizon Communications Inc.

Supplies were tight at those locations as well.

“We expect to sell as many iPad 2s as we can receive from Apple,” said Ravi Jariwala, a spokesman for Wal-Mart.

Trudy Muller, a spokeswoman for Apple, said the company was “working on getting the iPad 2 into the hands of every customer who wants one as quickly as possible.” She called the demand “amazing.”

The latest version of the iPad is thinner, lighter and more powerful than the previous one, and has a camera in the front and back for video conferencing, taking photos and videos. Its price ranges from $499 to $829, the same as the original model.

Firms that took apart the iPad 2 for analysis over the weekend found few surprises, with Apple mainly sticking to familiar component suppliers.

On one closely watched topic, the Ottawa-based firm UBM TechInsights concluded that Samsung Electronics Co. manufactured the Apple-designed A5 processor inside the iPad 2, continuing the pattern from previous Apple devices.

Some market watchers had speculated that Apple would shift to another manufacturing service because the Korean company competes with Apple in smartphones and tablet PCs.

Samsung and Apple both declined to comment.

Data-storage chips known as flash memory from Samsung and Japan’s Toshiba Corp.—both regular suppliers to Apple—were found in different iPad 2 units taken apart by UBM TechInsights.

The firm estimates that components inside an iPad 2 with 32 gigabytes of data storage cost Apple about $270, compared with $245 for the original iPad.

Because the iPad 2 is a refinement of the original, rather than a complete makeover, some analysts had expected the lines of consumers waiting to buy it to be shorter this time. Instead lines were twice as long in many locations, possibly in part because the company didn’t take pre-orders this time.

“We didn’t expect anything close to this,” said Gene Munster, an analyst with Piper Jaffray & Co., adding that Apple probably could have sold more iPad 2s if it had more of them in stock.

A survey of about 200 people conducted by Mr. Munster over the weekend found that most of those who bought the iPad 2 didn’t own the previous version.

With the launch in 26 other countries, including big markets like the U.K. and Japan, scheduled for March 25, Wall Street analysts are likely to boost their iPad-sales estimates of about six million units for the quarter ending in March.

Gleacher & Co.’s Brian Marshall said he believes his 2011 estimate of 30 million units is conservative though it is more than double Apple’s sales in 2010.

In the most recently reported quarter ended December, the iPad contributed 17% of Apple’s $26.7 billion in revenue.

RIM’s tablet which is set to hit the market before HP’s competing tablet TouchPad, reportedly uses the fast app switching which has been a hallmark of WebOS since it debuted on smartphones. Both tablets are believed to manage apps as cards, allowing users to swipe between them as they run in the background. The two tablets also reportedly have similar swipe gestures, allowing users to swipe apps off the screen.

Jon Oakes, director of product marketing, TouchPad, told Laptop mag on the issue “From what we’ve seen in the market, there are some uncanny similarities. It’s a fast innovation cycle and a fast imitation cycle in this market, so we just know that we have the creative engine here to continue to build on what we have, and we’ll keep innovating, we’ll keep honing and those guys hopefully will continue to see the value in it and keep following us by about a year.”
Jeff McDowell, senior vice president for business and platform marketing, RIM reportedly said in response, “I feel that we set out from the ground up to define a user experience that we felt would delight our customers, and we landed in a place that may look like other competitive devices. But there was no intention and no preconceived notion that this is what we want to end up looking like. In fact, I think QNX had that design lined up before we even started working with them.”

“You know, cars over time end up looking a lot alike because you put them through a wind tunnel, and when you’re trying to come up with the best coefficient to drag ratio, there’s one optimized shape that gets the best wind resistance, right? Well, when you’re trying to optimize user experience that juggles multitasking, multiple apps open at once and on a small screen, you’re going to get people landing on similar kinds of designs,” McDowell added.