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]
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).
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]