Well i dont know much about those fancy cell phone things, but i know rbcp pays for his thanks to his recent blog entry about sprint. Therefore I think its safe to say that its not possible to steal cell service in any reliable way.
but if you need to make a call and your in the middle of town with no cell phone, you could try finding a mobile phone store and use the demo phones for free.. most of the time they are on and working. I see people do it here all the time. its almost as though they are replacing pay phones with free ones... Although it does have to be during busness hours.