Apple's much-hyped iPhone 4 finally went on sale Thursday following weeks of pre-launch buzz.

The device is available at retailers Wal-Mart, Radio Shack, and Best Buy, as well as Apple and AT&T stores. Whether consumers can actually get their hands on an iPhone 4 is another matter. Supplies are expected to be short, and long lines were forming at stores around the country. Best Buy's Web site listed both the 16GB and 32GB models of iPhone 4 as "sold out" early Thursday.

Apple said it's attempting to make new shipments of iPhone 4 available as soon as possible. But the company conceded Wednesday that the white version might not be back in stock until the second half of July.

"White models of Apple's new iPhone 4 have proven more challenging to manufacture than expected," Apple said in a statement. "The availability of the more popular iPhone 4 black models is not affected," Apple said.

Demand for iPhone 4 is such that Ebay sellers have been asking for—and getting—prices that are marked up by 300% or more. One Ebay merchant sold a 32GB black iPhone 4 for more than $900 Wednesday.

Officially, the 16GB model is priced at $199 while the 32GB model is listed at $299. Both prices require a two-year AT&T contract.

iPhone 4 features a number of enhancements over its predecessor, the iPhone 3GS, including higher screen resolution (960 x 640 pixel technology dubbed Retina Display), a front-facing camera for video chats, and a longer lasting battery that affords 40% more talk time. It's also 24% thinner.

iPhone 4 also includes a 3-axis gyro and a slick new glass and stainless steel case—and it's capable of multitasking thanks to the new iOS 4 operating system.

"iPhone 4 is the biggest leap since the original iPhone," Apple CEO Steve Jobs proclaimed as he unveiled the product on June 7 at an Apple event in San Francisco.