A friend of ours wondered out loud the other day if it was worth all the water it took to clean out peanut butter jars to recycle them. Would it be better to trash the jar and save the water and energy to heat the water? This is a tough question to answer. Water and energy are (in theory, at least) renewable, plastic (unless it's made from plant oils) is not.
You can reduce the water needed by using a good brush and some elbow grease. I also head that you can loosen the peanut butter by tossing it in the microwave briefly. I thought this sounded like a good idea, so I tried it out today. I thought the addition of a little water would prevent overheating.
I learned two things today. Don't put the jar in for 2 minutes or the plastic will start to melt and you'll have a big puddle of peanut-buttery water in your microwave. Also, make sure you get all the foil seal off of the rim first.