Apple claims that over 100,000 developers have downloaded the iPhone software development kit (SDK) in just four days.
Apple said originally that it would not issue an SDK for the iPhone as it wanted to keep new software development in-house.
But after a wave of protests from developers and users the company reversed the decision and made an SDK available.
"Developer reaction to the iPhone SDK has been incredible," said Philip Schiller, Apple's senior vice president of worldwide product marketing.
"Over one million people have watched the launch video on Apple.com, further demonstrating the incredible interest developers have in creating applications for the iPhone."
AOL, Electronic Arts, Salesforce.com, Namco and Sega have already built applications for the iPhone using the SDK, according to Apple.
"We are very excited about Apple's new SDK and reaching every iPhone user through the new App Store," said Scott Rubin, vice president of sales and marketing for Namco Networks.
"We cannot wait to show off great new versions of arcade classics like PAC-MAN and Galaga that use the revolutionary features of the iPhone and iPod touch."
Apple's decision to open up the iPhone was generally welcomed by analysts. " The SDK is available as a free download. However, there is a $99 annual joining fee for the Apple developer programme," said Tony Cripps, analyst at Ovum.
"Of more interest is the distribution model Apple has adopted, which is based on a revenue share agreement with the developer.
"The developer gets 70 per cent of the revenue generated by sales of the application and Apple gets the remaining 30 per cent, and the arrangement is not dependant on sales volumes. This is a good deal for developers."
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