xlinknz: Just checked and I have incurred 0.09c of charges this morning I have only had the phone turned on 2 hours and not used it for anything
You haven't said what network you're on. According to Johnr you don't need the internet APN configured to send or receive pxt. I can confirm that I don't need to do so for 2degrees either. So if you're on either of those two networks you have a solution. Use an app like APNdroid to turn your APNs on/off from a homescreen widget. APNdroid has the option to leave the MMS APN configured so MMS will keep working. As far as I can tell from the Telecom APN settings this should work on Telecom too.