I saw an option when first setting up WP8.1, it decribes "Your cellular data won't be used if Wi-Fi connectivity is limited". For which this option can avoids accidentally data using, it always disables mobile data as long as your wifi is connected. Not very sure, but I think I saw it once under Data Settings if I remember correctly, however, now I cannot find it anymore at anywhere. Does anyone know this option, too? And know where this option is at?