You all know that some apps require "full internet access', and other permissions. Some use it for ads, some use it for other reasons, etc. So, can these apps access the internet when my wifi network and mobile network are disabled? I'm afraid of extra charges from my operator for extra MB. I mean those apps still show ads even if any network connection is disabled - how is it possible? In other words - how can I be sure if an app doesn't use any traffic while wifi and mobile networks are turned off?
Well, if you had your 3G off, then they can't access that, I suggest if you didn't override it, use APNdroid from the market to do so and it also features a good widget. Anyways, applications can't override your WiFi settings, if it's off, then they can't turn it on, but if you left your 3G on without any overriding application, like APNdroid, then they might use that, and this will ruin your data-plan.
I suggest you use 3G watchdog and APNdroid, both are available in the Market, to ride and monitor your data-plan, and for WiFi you got the power widget or any other mean to turn it on and off.
Keep in mind, keep your WiFi and 3G off all the time until YOU decide you want to be connected. And if you enabled 3G, turn automatic sync before turning your 3G on, sync lives over MBs if you got more than two accounts to be synced as I do.