I have observed this happening 3 times to me in the past month. On the iPhone , go into Settings > General > Usage and you may find that your Cellular Data Outgoing (3G or Edge) usage is showing up as 4Gb and incoming data usage set as 4.1Gb.
I have managed to save a screenshot of this weird iPhone cellular data glitch (see below)
Apparently this iPhone error is happening to many others and thankfully my Vodafone account is only showing a few hundred Meg being used and not 4Gb (or 8Gb as this is the second time this month that the data miscalculation has appeared on my phone).
Some (I did a bit of research on Mac Rumours website and there are others with this problem) are blaming one of the bits in the software which, out of a 32 bit system is causing the upper limit of the cellular data usage to be reported … or something along those lines.
Hopefully this cellular data usage glitch fixed by Apple soon in the next iPhone update…. either that or I really did use 4.1Gb!