Pent-up demand for the next iPad has sent customers clicking the “pre-order” button on Apple’s Web site in droves. In a statement to USA Today, Apple said that customer response to the new tablet has been “off the charts” and confirmed that the pre-order devices have sold out.

As of this weekend, buyers will have to wait two or three weeks to get their hands on the tablet — or brave the crowds at Apple stores. The lines for Apple launches are historically jam-packed. Because the company is rolling out the devices in Europe and Asia at the same time as the North American launch, the new iPad launch is on track to be one of Apple’s largest-ever device debuts. Apple, which has sold about 55 million iPad devices to date, is expected to sell at least that many by the end of 2012.

So it’s probably no surprise that Slashgear reports that they’ve already come across two people who’ve been camping out at London’s Regent Street store. According to the report, the two men — Ali and Zohaib — started the iPad line at the store on Saturday, packing folding chairs and warm clothes.

The new iPad, which adds a sharper display, faster processor, access to 4G LTE networks, a better camera and voice dictation to the tablet, will be in stores on Friday, March 16. The WiFi version starts at $499, and the 4G version starts at $629, not including the data plan.

The new iPads with 4G connectivity will be available on AT&T and Verizon, though analyst Kevin Smithen said that Verizon will probably see a bigger boost from the gadget because it has a more extensive 4G network, Bloomberg reported.

For those interested in getting the now-cheaper iPad 2, the tablet is still available in one to three business days on Apple’s online store. The 16 GB WiFi version of the iPad 2 now costs $399, while the 3G-enable version costs $529.

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