Yahoo! stock dropped 10% today, according to a post by John Battelle,  on news that online advertising growth appears to be slowing in the auto and finance sectors.  Some think this is a good time to buy Yahoo! stock as the advertising slowdown is attributed to sort term (according to Safa Rashtchy of Piper Jaffrey):

"Auto Advertising Slowdown Appears to Be Short Term. Our discussions with
a major auto advertising services firm suggests that there was a slowdown in new car launches in Q2 and Q3, which could have a modest impact on Yahoo’s Q3 revenues. That said, our discussion indicated that there should be a significant increase in new car launches in Q4 and 1H07. Additionally, we believe there was increased advertising activity for employee discount pricing in the summer of 2005 which did not repeat this year. Hence, we believe the slowdown in auto advertising referenced by Yahoo is primarily a seasonal effect and we would expect a reacceleration in Q4 and 1H07 on an annual basis – our checks indicate 30%-35% growth expectations for 2007. "

The other issue could be the development of Panama, the new Yahoo! AdServing platform that has been delayed.