First of all, cell phones do not emit nearly enough microwave radiation do such a thing. If that were the case, we would all have cancer. Secondly, that is not how a microwavable bag of popcorn works to begin with. Popcorn bags have a layer of foil at the bottom that is heated up and pops the popcorn: The microwave radiation does not pop the kernel itself.
I was babysitting some kids and they saw this on youtube and really wanted to try. Even though I knew it wouldn't work, it would still be fun for the kids. We took our four cell phones and placed them around the kernel just like in the video. I had four of my friends call at the same time and guess what? Nothing happened. Just thought I would fill you guys in.