After months of worrying that third-party iPhone apps would “take down” AT&T’s network or leave users perplexed as to how to use the device, Apple CEO Steve Jobs said an iPhone software development kit (SDK) would be made available by February.

Apple announced the SDK in a Jobs-signed note on Apple.com. Even without a formal tool kit, developers immediately began building native iPhone applications when the iPhone launched this summer. All sorts of apps not on the original iPhone – instant messaging, games, ringtone-makers, voice-over-IP, voice recording and more – were all made available by developers as native applications.

But with a software upgrade released last month, Apple not only closed up holes that let hackers unlock the iPhone for use on other networks, it also changed its underlying code to break the third-party apps that had been built up to that point.

The customer uproar about the lockdown seemingly wore Jobs and Apple down. The announced SDK will support application development not only on the iPhone but on the iPod Touch as well, which runs the same software as the iPhone without the phone capabilities.

Jobs said Apple was focused on opening the iPhone while limiting its exposure to mobile viruses and attacks, pointing to requiring digital signatures as one possible approach. “While this makes such a phone less than ‘totally open,’ we believe it is a step in the right direction,” Jobs said.

Up to now, Apple has encouraged developers to target the iPhone by building Web 2.0-style applications specifically targeted to the iPhone’s Web browser, which is built on the same code base as its Safari desktop browser. It recently opened a directory of browser-based iPhone applications at apple.com/webapps – including a mobile Facebook application and a banking app from Bank of America, among others.

Still left as an open question is the distribution model for third-party iPhone apps. Will Apple require applications to be distributed via iTunes – the only way to get Apple-approved music, movies and games onto the iPod today, as well as the required path for activating the iPhone – or will developers be able to distribute and sell iPhone apps directly? For now, Apple’s not saying. That would not only allow Apple to control the proliferation of iPhone apps, it could potentially take a cut of the revenue as well.

“The buy-and-load through-iTunes model seems to be deeply ingrained and also works very well,” said David Chamberlain, principal analyst-wireless at In-Stat. “If you accept that the preferable user experience would be downloaded, native applications rather than Web applications, then iTunes seems to be a great delivery mechanism.”