From November 3, 2008, until December 31, 2008, Yahoo! will be making a seasonal rate adjustment by increasing the cost per click (CPC) by 25% in all categories. This means that during this period your actual billed CPC will be 25% higher. This adjustment reflects the increased consumer buying activity during the holiday shopping season, which is typified by increased leads, better conversion to sale, and increased revenue for our merchant partners. The 25% adjustment to your billed CPC will be reflected on your Click Report and in your invoice.
Seems very strange to us here. Why not let bidders outbid each other? Why force the issue like this? Clearly, just to gain profits.
Yet another reason I prefer Google ads to Yahoo ads - the first being that the CPA is much, much lower on Google.