Hi
I also have an iPhone and the exact same thing happened to me.
I'm on a prepaid plan and for some reason, the credit dropped from some 800min to only 185min. Apparently, like the whole apn thing said in the previous post, push email or other stuff may be using the data. It doesn't even need to use 1mb, as long as you connect to the data line, it starts charging money.
No, I don't think they can charge you for using wifi, because isn't that illegal? They have no control over your device or using wifi, so it's not reasonable for that to happen.
You can install Data Disabler from Cydia to stop data usage. Also install the data toggle for SBSettings. Or you can upgrade your phone to 4.0 which has a built in setting to disable the data usage. The 4.0 option would be the best, because it survives resprings and restarts(the cydia option sometimes doesn't survive restarts). THe 4.0 option is also reliable, since it virtually turns off the data part of the phone