The Yahoo! Blog advises that they have a new web crawler:
“We launched a new Yahoo! Search Crawler, Yahoo! Slurp earlier this week. In addition to crawling the Internet faster, our new crawler is more efficient at visiting websites. As a result, site owners will notice as much as a 25% reduction in the number of requests and bandwidth consumed by the crawler”
If you see some changes in the Yahoo! serps, it may be a result of crawler activity.