Cellphone to Arduino
You can find a Motorola C168i on Ebay for less than 10 dollars. The simplest way to get service for it is to buy a pay-as-you-go SIM card, at Radio Shack for example. While you're there you can also pick up the 3/32 plug; unfortunately, they don't have any information on the packaging but I've included a figure here for your reference (see left).