Yahoo and Bing partnered up in the beginning of the year, and now Yahoo is currently experimenting with Bing results. Most Yahoo queries show Bing results, and is very possible that at the end of next month, those results will be permanent.
If Yahoo! will use Bing data, then Yahoo's own crawler, Slurp, won't be crawling and Bing will get more famous. The both internet companies want to create a search engine that can come in competition with Google, the largest internet company, and they both have agreed to join their forces to take down Google, which is unlikely to happen.
Some webmasters have issues with Bing crawling their sites and most of their sites are not fully indexed . In order to come in competition with Google, Bing will have to improve its searches and deal with webmasters individually, which can take up to 5 years.