Well I’m not sure how much this figured into the Apple’s plans when the released the iPhone SDK but one of the interesting benefits that Apple will get out of all this interest in developing iPhone apps is a signficantly expanded Mac developer base. This may have some interesting ramifications for the viability of MAC as a mainstream platform.

Let me explain – despite all the new conventional wisdom about the the web being the next operating system, there are still many mainstream computing applications that are run as native client-side code. Consider, word-processing (Word), spreadsheets (Excel), music player (iTunes), chat (YahooIM, Skype), and P2P (BitTorrent and Kazaa) and gaming are all still run predominantly run as native apps be they Windows or Mac apps. One can still make the case that that health of a platform can be measured by the number of developers it has.

If that is true, the ability for the MAC OS platform to maintain and grow market share will be dependent on the number of OSX developers Apple can bring under its stable.

Luckily the iPhone SDK does just that. You see, the iPhone is essentially a little palm top computer that runs a stripped-down version of Mac OS on a computer with a slower CPU and smaller memory. iPhone developers will write iPhone apps using the same development language (Objective C), tools (XC0de) and writing against almost all the same API’s (COCOA, Mac OS X and BSD functions) that are needed to write a MAC program. To writ, by learning how to write an iPhone app, you teach yourself how to write an Mac OS app.

The net result – a well trained iPhone developer will automatically know about 85% of what is needed to write an Mac OS app. Given the 100,000 iPhone SDK’s that have been downloaded in the 4 days since it was released and the thousands more can be expected to , how many of those do you think will take a peek next door and try their hand at developing Mac OS X Apps?

Granted I doubt we will see an multi-billion dollar MAC only software company spring from all this. But what we will see is that with a increasing number of trained MAC developers, existing software companies will have a larger pool of talent to port their existing Windows only applications to Mac OS X. This of course kicks starts the fabled “virtuous circle” of network effects we see in technology industry – more programs for Mac means Macs increase their attractiveness mean more MAC users means more programs and so forth.

One other thing is that we should consider is how easily iPhone apps can be ported to MAC. It’s interesting that apps usually start life as a desktop app before migrating to a mobile application. However, with all the VC $$ pouring into iPhone applications (witness KPCB’s $100m iPhone fund), and the fact that users want applications that span all their devices from phones to laptops, it’s not inconceivable to consider the day all those iPhone apps start-ups might offer a desktop version of their product to the MAC OS first just given how easy it would be deliver this. Simply recompile (if even) and go. Do you think an MAC first release might sway folks on the fence about their laptop OS to consider switching? It very well might.

So there you have it. Apple’s iPhone’s SDK as a brilliant strategem to increase the attractiveness and viability of the MAC OS in Apple’s home computing segment – where Apple still earns the majority of its revenues and profits. Apple is already a $110B company with only 2.8% worldwide market share, albeit heavily skewed in more pricey segments of the computer market and in North America.

That iPhone will continue to drive demand synergies for the MAC (Apple is predicting 10M iPhone sales by next year and has already sold 100M iPods) and now supported with an swelling ranks of MAC developers only increase the favorable picture Apple can expect to see in its core business in coming years.

 Appended: I forgot to mention one other thing – the iPhone SDK will power MAC sales.  This is because the SDK (surprise, surprise) only works on a Mac.  Have a windows machine?  You are out of lunch.  So, conservatively assuming even 1/3 of the all iPhone SDK downloaders are Windows switchers, that’s another $36M -$75m in the bank depending on which Mac they get (Let’s say somewhere between a Macbook and a mid-range MacBook pro)…