The Yahoo Search Marketing Blog announced two main changes that went into place yesterday.
(1) The most important change was that a new pricing algorithm is in place. Yahoo said, "we're now expanding the adjustments we make to our click charges based on our assessment of the performance of traffic coming from sources within our distribution network." Yahoo added, "based on our analysis, we expect that most advertisers will see click charges drop or remain unchanged as a result of this change, while a small fraction of advertisers may experience an increase in click charges."
(2) Yahoo added a report to weed out how Yahoo's partners are performing, which gives you the tools to block underperforming partners. The report is named the Ad Delivery Report and it allows you to "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."