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?"