The way I fixed this on my jail broken 2g iphone (back in the 2.2.1 days) was to go into settings->general->reset->reset network settings (then select reset)

Once that was done (I think it restarted or required one), the option to change my apn settings was there under settings->general->network->cellular data network.

I had tried the cydia apn hack and various other things but in the end, after a ton of nail biting and hair pulling, this is what worked. I hope it does for you too.