It's going to happen - probably without Pinterest before it will with their blessings. Did I miss it? Has it already happened?
I can't help but think some current Facebook engineers and/or ex-Google product managers (who could be one in the same) trying to figure out how they build the next big third party ad platform right now.
If I were a little more organized and capitalized (i.e., perfect for an incubator), this is what I would do - I, being a hot incubator, would use my clout and prestige and create a competition looking for the best Pinterest-based ad network. Winner gets funding. I would keep it a closed competition and fund the ten best ideas / teams.