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There's growing speculation among Apple (AAPL) watchers that the iPhone software developer's kit (SDK) that Steve Jobs said he hoped to have ready before the end of February — and is the headliner of the "special event" scheduled for March 6 — may still be in beta and might not arrive until the World Wide Developer's Conference some time in June.

This would be a surprise, although probably not the "element of surprise" that COO Tim Cook invoked when he declined to answer questions about the SDK at Wednesday's Goldman Sachs symposium. (see here)

What's the source of this speculation? Some of it is pure gossip, like the item the usually reliable Erica Sadun at TUAW passed along earlier this month:

Speaking of the rumor trickle, the big one that I'm hearing right now is about big SDK delays — perhaps all the way to WWDC (which makes us think that the big Apple show coming up may not be for the iPhone). It sounds like putting together a public SDK, documented and tested for third-party use, is a huge, huge project, and that Apple is busy hiring people to make this happen. (link)

Some of it seems to be coming, albeit second-hand, from programmers who actually laid hands on preliminary versions of the SDK, like this report in MacRumors Thursday (pointing out an item in the Italian website setteB.it):

SetteB.it is not a typical rumor source, but this information is consistent with earlier Page 2 rumors and direct observations we've heard from individuals who have seen early versions of the SDK, saying that it appeared to offer just the "bare essentials" at that time. (link)

Some of it is just tea-leaf reading, like Ted Landau's close scrutiny of Apple's iPhone Software Roadmap invitation in The Mac Observer.

Why use the word “roadmap” at all? Why not simply say: “Please join us to learn about the iPhone SDK” or “Please join us for the unveiling of the iPhone SDK”? (link)

Some of it is Fake Steve Jobs spinning fiction, as he usually does, but often around a kernel of truth:

Look, I've gotta admit, I'm pretty disappointed with the developer tools engineers. These guys have really let me down. We were supposed to have the iPhone SDK out by February and let's be honest — it ain't gonna happen. … Damn you, engineers! If I could run this company without you, I would do it in a heartbeat! We'd have nothing but PR and marketing and advertising people. (link)

Whatever SDK beta meme's provenance, some investors have already figured it into their planning. The smart money at TMO's Apple Finance Board is betting that come March 6, as Tommo_UK puts it, …

… the big news will be software partners signing up, big names like IBM, SAP, Salesforce, Microsoft, etc.. some with demos etc. A beta of the SDK will be released to allow smaller developers to play, but it will run in emulation mode on the Mac rather than the iPhone.

The main SDK will be released concurrent with a new iPhone or iPhone OS 2.0 instead of 1.1.6 or whatever, and at that point the entire development community, rather than just the "chosen few" keynote partners will be able to write and sell applications for the iPhone. (link)

UPDATE: One more datapoint. Jeremy Horwitz at iLounge, citing unnamed "sources familiar" with Apple's SDK plans, has published an authoritative-sounding rundown of how Apple plans to restrict third-party developers writing apps for the iPhone and iPod touch. (Apple would have to approve all apps, for example, and would prevent interfacing directly with dock-based accessories.) He adds that his sources say this about what's coming next week:

Apple will use the March 6 event to tout the benefits of the SDK to selected media, analysts, and developers, releasing an incomplete, “beta” version of the kit that was originally promised for February. The actual kit will now ship in June, coinciding with Apple’s annual Worldwide Developers Conference event in San Francisco. (see here)

UPDATE 2.0: Apple released beta version of its SDK today. The final release is due in June. See here.