I love this idea, but would also suggest "Smart Pots," (www.smartpots.com) which are a fabric container. To plant, you only need one more step, which would be pouring the potting soil into the smart bag. They come in all different sizes and are pretty inexpensive. They let the roots breathe and are made of recycled materials.

I know that some plastic (pvc) leaches lead when exposed to the sun, (garden hoses, for example, but that's a whole other story). I'm not sure if the plastic from the soil would break down.