In August, Yahoo announced a new crawl behavior for Slurp, Yahoo's web crawler. The new crawl behavior was suppose to tame the crawler to go through your site in a more relaxed and efficient manner for both the crawler and your server.
But it seemed like soon after, Webmasters were not happy with the new crawler's behavior.
Recent discussions over at DigitalPoint Forums reports that Yahoo Slurp seemed to have taken it easy on some sites. It is hard to confirm that this is a global change, since only about four or five Webmaster have confirmed this activity, but it does seem like something may be up at Yahoo.
Forum discussion at DigitalPoint Forums.