Yahoo just announced some new additions to their PPC reports including the ability to see what partners are sending you traffic.
Their help section details some of it here:
With the Ad Delivery Report you can view information about the URLs where your ads are being displayed. You can then use this information to help inform your decisions regarding which domains to block using the “Blocked Domains” feature.
The Ad Delivery Report allows you to view:
* a list of domains and specific URLs where your ads were displayed
* performance metrics for your campaigns in terms of impressions, clicks, cost, conversions, etc by URL (Analytics must be installed to track conversion data. Learn more about installing Analytics)
* information about how certain URLs are performing, potentially enabling you to make better decisions when using the Block Domains feature