settings, usage, cellular usage, cullular network data
probably ios 6.1.2
but when the iphone is on standby for a while. and you wake it up, it is on 3g and then switches to wifi, this never happened with the iphone 3gs or iphone 4, or at least didn't, anyone with iphone running ios 6.1.2 want to verify?
the problem is that everytime it does that a little bit of data is sent, not an issue for those on unlimited data, bundled data etc, but for those without or pure payg, that will eat into the data usage/credit
you could switch of cellular data, but that would mean switching it on everytime you needed to use it