Very interested to see that someone else has had the same problem.
Travelling abroad in December, I bought a data roaming bolt-on from O2 but kept data roaming switched OFF until I absolutely needed it. A few days later, the iPhone showed that a few tens of kB had been transferred. Very strange.
Interestingly, when my bill arrived from O2, I'd been charged for the trickle of data, rather than including it in my roaming package. Curiously, the bill shows the "source" of the data as "idata.o2.co" (ie a nonsense address, missing the .uk at the end), whilst all my deliberate roaming, later in my trip, shows "idata.o2.co.uk".
It gets worse though. I used an airport wifi service to download my email without eating into my roaming package. When the airport wifi connection dropped, the iPhone automatically failed over to the cellphone network and downloaded several weeks of email all in one go. Data Roaming was still set to OFF. This absolutely should not be possible. This time around, my bill does show the full "idata.o2.co.uk" as the source. But it still shoudln't have happened.
The very same thing happened again yesterday while travelling abroad - wifi was lost and the iPhone failed over to the cellphone network and started data roaming, even though Data Roaming was set to OFF and has been for nearly a week.
On both these occasions, I was definitely trying to access the internet, and was using either Mail or Safari to do so. However I was doing it by wifi, and both times data roaming was OFF.
I've brought this to O2's attention. But is anybody seeing this on any other networks? And is there any official word on what's going on?