Depending on trigger source the power consumption should be next to zero.
E.g. the accelerometer has a built-in IRQ generator that engages a special hardware IRQ line even for doubletap - you can configure the parameters like time distance between taps, impact strength, and impact direction (snip against lower edge of device, tap on backside, etc). Unless this doubletap event gets detected by accelerometer, the whole system is as idle as it's been without the gesture daemon running.

Alas especially the accelerometer needs a kernel driver patch (http://talk.maemo.org/showpost.php?p...45&postcount=8) to make this doubletap thing work, so this might be restricted to powerkernel users.

Other sensors like switches/buttons, sliders (camdoor, kbd), proxy sensor, headset button, etc will usually work though, and should be equally humble about power consumption.

/j