With all of the talk of software, there is another major aspect to the "software" developers kit: HARDWARE, specifically non audio-out hardware, more the type of hardware that only will work for the touch and iphone.
While many 3rd party applications have already been written for jailbroken iphones/ipods, they are somewhat limited due to the fact that they're not Apple sanctioned. Hardware is even more at a loss, because few companies are going to invest in making a product that could be made useless at any moment and attacks Apple's 3rd party positions.
However, being able to write 3rd party apps could open up this arena. Hardware, and the software to run in with the SDK, could bring powerful new options to the iphone/ipod touch.
Personally, I'd like to see a portable keyboard so that taking notes on the ipod/iphone is as easy as on a laptop, just far more portable. I'm not going to go into the endless hardware options out there (feel free to post more on that below), but we all know they're out there.
So, lets hope for that the SDK allows for a product we KNOW won't be for free, hardware.