I figured out how to do this a while back:
if you have an old, non-smartphone lying around (in a pinch, someone else's will do), then all you need to do is stick the sim card into that phone, get the unlimited dataplan for NON-smartphones (i think it's called something like medianet), then stick it back into your iphone. If you're on a family plan, you can reduce the price to $20, or potentially lower, for unlimited data. I managed to get a $10 unlimited data plan on my iphone 3g using this method.
if you have an old, non-smartphone lying around (in a pinch, someone else's will do), then all you need to do is stick the sim card into that phone, get the unlimited dataplan for NON-smartphones (i think it's called something like medianet), then stick it back into your iphone. If you're on a family plan, you can reduce the price to $20, or potentially lower, for unlimited data. I managed to get a $10 unlimited data plan on my iphone 3g using this method.