Anyone who's familiar with Rocket Fizz will know that this isn't a novel idea. I'm pretty sure I've seen (but never tasted) both a peanut butter soda and a pb&j soda from them, though the reviews weren't all that great. If I remember right, the complaints stemmed from the fact that they tried to give theirs some sort of creamy, thick mouthfeel. I think this should be something that you don't need to drink with a spoon, so I went the powdered peanut butter path. I've heard of homebrewers using this for their peanut butter porters because it has a low oil content and mixed well with water.
Basically it's ground nuts that most of the oil has been pressed out of, so if this gives you the impression that you're mixing up some artificial "just add water" stuff, then you are gravely mistaken. This is all natural "just add water" stuff. And if you had a blender, and an efficient way to press 85% of the oil out, you could probably make your own.
Anyway, when I saw this on the shelf at the local Wal-Mart and discovered that I wouldn't have to order from Amazon, I knew that peanut butter soda was in my future.
2 cups sugar
1/4 cup PB2 Powdered Peanut Butter,6.
2 cups water
1 gallon carbonated water
Bring the water to a boil and add the peanut butter powder and remove from heat. Stir and allow to cool. Strain out the larger peanut granules using a fine mesh strainer. Add the sugar and heat to dissolve. Mix the resulting syrup with 1 gal carbonated water or to taste.
It worked out well, I was pleasantly surprised that it wasn't too oily or gritty. There's enough fine powder to get a nice smooth syrup with minimal filtering. It could use some jelly flavor, or possibly an addition of chocolate flavor.
Now, there are likely easier ways to do something similar. Watkins makes an artificial peanut flavor extract which would probably get you the same effect. But that's kind of cheating, unless you happen to be allergic to peanuts, and then it's probably a good idea.