Note this will not keep them from charging high rates for datatraffic, or setting very low caps, and charge lots more if you go over your allotment. Has cost me hundreds of euros per month for several months.
My iPhone appeared to be very uninformative about which apps were the data hungry culprit, and Apple has blocked API's for third-party developers. Also it seems that when you enable sending diagnostics info to apple, crashdumps will be sent AT NIGHT OVER 3G EVEN IF YOU ARE AT HOME ON WIFI!
My Dutch provider KPN was unable to offer any insight into my traffic, and was unable to help me with determining why I was consuming so much traffic.
Many ad-supported apps do not have switches to disable ads-over-3G, my traffic app was eating into my monthly
Overall I have been very disappointed at my iPhone in this respect, and no, I will not switch to Android yet, but this was a serious downer.