Yahoo is launching a new pricing system. It is what is called “Smart Pricing” in Google Adwords. Here’s a copy-n-paste of their email:
Quality-Based Pricing Launches

In an ongoing effort to raise the value of our Sponsored Search and Content Match products, we’re pleased to announce the phased rollout of a new feature that we believe will help increase the value of Yahoo! Search Marketing traffic to our advertisers. This feature, called quality-based pricing, is designed to measure the value of the traffic coming from our distribution partners’ websites and price clicks accordingly for our advertisers.

What is Changing?
Previously all traffic was treated the same—you were charged the same for traffic from all web sites within our network. With quality-based pricing, you may be charged less for certain clicks than you normally would pay, depending on the overall quality of the websites that are providing this traffic to you. As a result, your click charges can decrease.

How will Click Charges be Discounted?
“Quality” is calculated based on conversion rates and other measurements of the ability to deliver more interested and valuable customers to you from particular distribution partner sites. Discounts will be automatically applied to your account.

When will this Start?
This feature will roll out beginning today, and we plan to continue to expand it over time. This is just one of the things we’re doing, along with future projects like domain-level blocking, to help increase the quality and value of traffic that we deliver to you.

What Do I Need to Do?
There’s nothing you need to do to receive a discount—your click charges may be discounted based on the quality of traffic you receive. Keep in mind that the amount of the discounts may vary between advertisers: Some may experience a noticeable decrease in overall cost-per-click, while others may experience only a small decrease in spend.

Do I Need to Change how I Bid?
Quality-based pricing does not change how you bid on particular keywords. As we announced in February, both bid amount and ad quality now determine an ad’s rank in search results. As always, you never pay more than your max bid.