Even if there are many questions regarding Android and GPS my doubt is really specific to the user level location settings.
Supposing I've enabled the GPS settings, say from the Power Control widget, and no application is actually requesting location updates.. what is the impact of this situation on battery life?
Does the settings location flag start some activity in background, maybe a location service?. Is some radio activity initiated by having the flag on? or can I assume that if no app will register for updates my battery life will be more or less the same?
I don't know the actual impact, since a lot of applications use the location-service (either GPS or just phone-antenna)
But you can test this by using CurrentWidget for example. This tool will give you the ability to log the battery usage at an interval.
Normally your battery life will be the same. Only if applications start requesting GPS updates the battery will be affected. The switch from the Power Control is just a shortcut to prohibit / allow applications to enable GPS location updates.
It should not have any impact on your battery life if you don't use any GPS enabled application. However there is nothing that forbids apps to use the GPS in background if it is enabled.
It is the case with Google Latitude. So if Latitude is enabled it will use the GPS in background in some occasions to find your location. In this case you will see an impact on your battery life.
Don't know of any other apps that does that.
Menu | Settings | About phone | Battery use will show you what has been using your battery.
I very seldom see GPS in there, unless I've been doing a lot of heavy Google Maps work, and even then not that much.
Try keeping an eye on that and see how much battery your GPS is really using.
Enabling GPS does not actually activate GPS. It just allows applications to activate the GPS. So leaving the GPS flag on all the time will not consume battery power unless applications activate the GPS and ask for the current location.