Do you have emails setup? are they set to push or fetch. It might be pulling on a quick schedule. Does she play any games maybe the game is still pulling info in the background? This is why I love the iPad once the data is up for the month bingo it shuts off no overage charges. I think that is such a crappy thing they do, you should atleast get a f'ing warning when you come close to your limit. You should tell your wife to turn the Cellular data and 3G off whenever you guys are in Wifi area this will make sure your phone only uses Wifi for any data usage.