Forum member "ttw" asks:
"Our client has affiliates that want to bid on many of the same keywords that we are bidding on. How can we manage this so that the affiliates don't drive up our cost on PPC?"
The resulting conversation helps answer this question, but also provides a ton of additional info on the pros and cons of having affiliates.
It's definitely worth a read:
How to Prevent Affiliates from Driving Up PPC Costs