Lycos along with Hotbot have ditched the Yahoo results and started displaying Google Adwords. Lycos may be Teoma content, not sure. I guess they decided the Google Adword revenue was better than Overture/Yahoo revenue. In fact Yahoo may have decided the Adword revenue was better than their own. One day you're nowhere to be seen next day you're back on top. I guess Yahoo's popularity is growing and growing.
Yahoo search has got better over time, so I dont think thats the reason why they drop them, I think it is a money question, Ok yahoo still misses a lot of sites, be cause of the only index page indexed, still I like them.