tourist:
All electronic devices that do not have a physically moving on/off switch which sits in different positions when on or off, not just a button, are never truly off. They have to have residual circuits on to recognise the button press to turn them on.
The only way to truly turn off any mobile is to remove the battery, and this is not possible to do with an iphone.
Apple tech support would beg to differ with you. If you hold the power button down long enough to get the red slider, then slide it, the unit is indeed fully powered down at that point.