[mattparcher]

As you may already know, you should be able to hold the 4S up to your ear, so at least it looks like you’re talking to another human, and Siri won't speak aloud. (I have not actually used a 4S, so my apologies if I'm mistaken.)

[joebadmo]

This doesn't change the fact that she has to talk out loud. I don't like to take personal calls at work, either. I usually go out into the hall or somewhere more private. I don't think this is rare behavior.

[neild]

It doesn't talk out loud if you hold the phone up to your ear. To use Siri with minimal noise (aside from your own speech):

Push the power button to turn on the screen. Hold the phone to your ear. Wait for the be-beep sound, played quietly through the ears-only speaker. Speak. Keep the phone against your ear. Siri will respond through the ears-only speaker.

[bretthoerner]

That's exactly what the parent said he doesn't want to do.

[neild]

Aha! I misread the parent's "she" as referring to Siri, not as to the person using the phone. Apologies.

[kyleslattery]

You're correct. I tried this with my girlfriend's 4S this evening—if the screen is on and you hold the phone to your ear, you will hear the ding prompting you to start speaking.

[chaz]

Battery life reportedly improves when this feature is turned off, due to Siri's constant monitoring of the accelerometer.

[WiseWeasel]

Minor nitpick; it's the proximity sensor which is used for this function, not the accelerometer.

I can verify that my iPhone 4S is polling the proximity sensor continuously, activating Siri whenever the phone is brought to the ear, while my 3GS only appears to poll the proximity sensor during phone calls.

[Edit] After further testing, it does seem to be a combination of gyroscope and proximity sensor, as you do need to perform the motion of bringing the phone to a position with its front facing somewhere between sideways and upwards and its bottom somewhere between straight down and sideways on its edge while covering the upper portion of the device to activate Siri. It does not work if you're laying down horizontally on your back or on your side, nor when you bring it to a straight vertical for example. It seems to have a fairly narrow tolerance for the angle to which the phone must be brought in order to activate Siri, that being the angle at which you naturally hold a phone to make a call while sitting or standing straight up. [/Edit]

[acchow]

I'm pretty sure the accelerometer has to be monitored continuously, with our without Siri. After all, how do you think geo-fenced reminders work? The mostly-low-powered accelerometer can wake up power-hungry location services to see if you've left your geofence. There's no reason to have the GPS running all the time (and your battery life would suffer significantly from that).

[gujk]

Why would it monitor constantly, and not just when you are using it, like how the phone behaves during a call with the face-disables-touchscreen?"