Too bad an open iPhone wasn’t the truly big news. Tucked in at the end of Jobs’s letter is the real bombshell: Apple will let developers write software for the iPod touch.

Who cares about the iPod touch?

A lot of people, it turns out. The $299 gadget has quickly become the second most popular iPod in Apple’s lineup, behind the iPod nano. Because of the way Apple’s sales have been trending, the iPod touch is probably more popular on a unit basis than even the iPhone (which is almost identical to the iPod touch except that the iPhone has a camera, a built-in microphone and speaker, and cellular service). The iPod touch also has greater reach – it’s available worldwide now, unlike the iPhone, which is just beginning to show up in Europe and isn’t scheduled to arrive in Asia until next year.

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So for software developers, the open iPod touch actually will be a bigger deal than the open iPhone.

Here’s why: Like the iPhone, the iPod touch is essentially a pocket-sized Mac with a touch-screen interface and built-in WiFi connectivity. Right out of the gate, developers are sure to begin offering nifty programs such as games, address books, e-mail organizers, and social networking utilities. But because the iPod touch in its current form doesn’t have cellular service and doesn’t have an external speaker or microphone, it’s a different device in some key ways. So many developers will write applications that don’t require cellular service, don’t require a camera, and don’t assume the presence of a built-in speaker and microphone.

Of course, programs that work on the iPod touch will also work on the iPhone. But experienced software developers know that it’s wise to focus first on the largest market – which is often the cheapest, most widely accessible device. In this case, that’s the iPod touch; the entry-level version is $100 cheaper than an iPhone, and as I mentioned above, it’s available worldwide. Developers also have every reason to believe that Apple will eventually update most of its iPod line so that the devices run on OS X, like the iPod touch and iPhone do now – and that means millions of potential customers for the programs they write today.

This isn’t to say that the open iPhone doesn’t matter. Many developers are sure to write applications that specifically take advantage of its always-connected abilities. But the open device that’s likely to have the bigger impact isn’t the iPhone … it’s the touch.

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