The Apple Headset will be fine. It's just like the current headsets. If you hold down the control button, you can access voice control right now too. Same idea, but now it's 100,000x more sexier with Siri.
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If u use a bt headset u activate Siri by pushing the button on it. Simples!
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It works with Bluetooth headsets so you can leave the phone in your pocket and communicate with Siri simply via your Bluetooth headset. However it is NOT required. Check out the Siri vids on Apple's website.
*presses bluetooth button*
"Siri, have a large pepperoni pizza from Donatos delivered to my home"
"Ordering now"
[30 minutes later]
Ding-Dong
*wife answers door*
"Honey, Domino's is here"
"NOOOOOOOOOOOOOOOOOOOOOOOOOOO"