Thanks for sharing this. I am interested in doing this with a prepaid SIM card that charges per txt and MB of data. As such, I want to eliminate unwanted background network access– e.g. even when I am not using my Macbook, there are a lot of small internet accesses that I can snoop using netstat. Is this a problem with the Raspberry Pi? I want to leave the RPi on 24/7 and only plan on sending ~5 SMS messages per month at 10 cents each and am worried that the low level data usage will overwhelm the savings from $10 per 3 mo plan.

Could you possibly run netstat to snoop your internet connection for a while and give me an idea of how much background activity there is?