MacRumors had an interesting article
earlier today about iOS 5's new "Raise to Speak" setting which launches the Siri personal assistant app when you hold the iPhone 4S up to your ear. The default way to launch Siri is with an extended press of the home button. Both feel like acceptable ways to interact with a first-gen voice interface.
Later in the day I put a call to my bank on speaker phone and noticed how I was holding my iPhone as I navigated their voice tree ("PUT ME THROUGH TO A GOD DAMN REP" isn't a valid option in case you were wondering). I was holding the phone upside down, talking directly into the mic just like I'm doing in the picture. Whenever I talk *AT* my phone, this is the most natural way for me to hold it. To prevent accidental activation when the phone is in a pocket or bag, require a quick tap of the volume button when the phone is turned upside down. Apple recently repurposed a volume button to act as shutter release in the camera app so using it for something other than volume control might not be out of the question.
I also thought about having the iPhone listen for a specific voice command when turned upside down, perhaps "Siri".
::flips phone:: "Siri, do you think Apple would have users say your name every time they want to interact with you?"
"Probably not." It has a very robotic, sci-fi feel to it, something you'd expect to see in a Droid Bionic commercial.
Apple is really good at training users slowly transitioning user behaviour so expect Siri to get a more natural feel over time.
::flips phone:: "Siri, is this an acceptable way to end the post?"