Rocket Fizz has (or had at one time) a Peanut Butter flavor. Never tried it myself, but read about it on the Weird Soda Review
Quite an amusing review, though not favorable by all accounts. There is also a PB&J soda.
Check Homebrewfinds.com for their link to the powdered, de-oiled peanut butter. That might be a good start for a soda recipe. I also believe you can get a peanut butter flavor from Walmart under the Watkins brand.