I've been working on iPhone development for several years and I have been very excited to see how the iPhone SDK has progressed. See my original article on how I envisioned the iPad revolutionizing the computing experience. Siri takes the iPhone platform to an entirely new level. Feature article by MD
If you're curious as to how the potential of Siri will impact iOS App development, then look no further than the video Apple used to demonstrate the technology, despite its somewhat heavy-on-the-awesome slant. The fact of the matter is, this is voice recognition technology that's approaching the level of sophistication you saw in Startrek Voyager as a kid, and it could do wonders for 3rd party App developers.
"Load Angry Birds. New game, please. Aim 45 degrees off the starboard. Lower 3 degrees. Fire. Raise 3 degrees. Fire. Down with the piggies! Fire."
It sounds far fetched, but is it? Voice control to do anything from text messaging to calling, and it's not difficult to imagine it controlling things that are not traditionally based around natural voice input. It'd also be a great idea to work in some camera/augmented reality games, as playing "I Spy" with Siri could only make the device even more intuitive, when you're not ringing your husband, friend or O2 without even touching your handset.
An open API to Siri will help push developers in a new direction building innovative software for the iPhone 4S, given that Siri pushes the boundaries in a way that encourages people to try something new. Google Goggles was a great example of this - it took what QR codes were doing and pushed it farther than anyone had expected it to go - I certainly didn't expect to be able to use my voice to dictate a text message, make two calls and send an email to my boss, did you?
The best thing about it is the potential it awards developers working with voice and hardware interaction, as unless you're paranoid about SkyNet and HAL-9000 becoming a reality, it certainly puts a friendlier face on an already friendly looking phone. All that's left is for us to enjoy the adventures many app programmers are embarking on with Siri, in addition to the talented invidiual who's just ported it to the iPhone 4. Good times to come, ahead.