In the mean-time, I found a couple of cool Internet radio applications and began using them at work and home - where I have access to Wi-FI. Today I checked my 3G Internet usage and found I have a bill for $256.97 for 125.48MB excess beyond my 50MB per month (excess Usage 0.20¢ per KB). I rang my supplier to explain the situation and asked to be forgiven for the debt. Politely enough I was refused and given a $20 discount instead.
It was explained to me that the iPhone has a feature: if the Wi-FI gets disconnected, it automatically switches over to 3G. I was horrified; under normal situations I would think this a cool feature, but not while I have only 50MB a month!
So here is my lesson; maybe you can learn it too.
How do i know if my iPhone is using Wi-FI or my Data Plan? On the top left of the screen, to the right of your supplier's name is an icon. This symbol means you are currently using your 3G data plan:
This symbol means you are currently using a Wi-FI network:
See page 21-22 of the iPhone User Guide to see what all of the possible icons mean - including EDGE and GPRS, which are other forms of Internet connectivity offered by certain suppliers.
Great - but it didn't save me, since obviously my Wi-FI disconnected at some point and automatically switched over to my 3G data plan. I have since found that you can stop that from happening too (iOS 4, iPhone 4): go to Settings > General Network > turn off Mobile Data. This will ensure that from now on, I will only be able to use Wi-FI for Internet - at least until my 1GB plan kicks in.