In an attempt to show how much interest there is in the iPhone SDK, Apple said the free kit was downloaded more than 100,000 times in the four days following the March 6 release. In addition, Apple (NSDQ: AAPL) said more than 1 million people watched a video on Apple.com summarizing the launch. But perhaps a better way to judge the interest is how much it has already been hacked: Within six days, “the beta 2.0 firmware that comes with the iPhone SDK has been modified to unlock the iPhone and run any application,” Gizmodo reports.

The iPhone Developer Program is currently available in the U.S, and will expand to other countries soon. In June, Apple plans to release the official software update to iPhones and the corresponding App store that will be the primary destination to buy applications. At that time, it’s expected that developers will have to pay $99 in order to participate.