Now, with only five business days before Apple’s (AAPL) self-imposed February deadline, Arik Hesseldahl reports in Byte of the Apple that it ain’t gonna happen — at least not on deadline. He writes:

I’m hearing from one source that its going to be late. I’m not yet hearing any reasons why, and it’s sounding like the official release date could slide by anywhere from one to three weeks. (link)

Hesseldahl, it should be noted, was one of the Businessweek reporters who broke the SDK story back in October, one day before Steve Jobs announced that it was coming in the Hot News section of Apple’s website.

So what’s the hang-up? Hesseldahl doesn’t speculate, except to say that the situation is “fluid” and that there are “a lot of moving parts to something this complex.”

But to get a feel for what’s involved, you don’t have to look any further than Jobs’ Oct. 17 letter, the one that starts:

Let me just say it: We want native third party applications on the iPhone, and we plan to have an SDK in developers’ hands in February.

Jobs goes on to say:

It will take until February to release an SDK because we’re trying to do two diametrically opposed things at once–provide an advanced and open platform to developers while at the same time protect iPhone users from viruses, malware, privacy attacks, etc. This is no easy task. … As our phones become more powerful, these malicious programs will become more dangerous. And since the iPhone is the most advanced phone ever, it will be a highly visible target. Some companies are already taking action. Nokia, for example, is not allowing any applications to be loaded onto some of their newest phones unless they have a digital signature that can be traced back to a known developer. While this makes such a phone less than “totally open,” we believe it is a step in the right direction. We are working on an advanced system which will offer developers broad access to natively program the iPhone’s amazing software platform while at the same time protecting users from malicious programs. We think a few months of patience now will be rewarded by many years of great third party applications running on safe and reliable iPhones. (link)

If Cupertino’s best developers haven’t cracked that nut in the four and a half months since, it could take them more than Hesseldahl’s one to three extra weeks. We’ll find out soon enough.