my understanding is,they just signed the deal,but it may be months before all the details are known. MS will handle the search engines and yahoo doing the advertising. if yahoo went powered by Bing,it would really be "if Bing has my site...". the answer would be yes,but if they work on yahoo,it could be still two different search engines. i don't keep myself updated on the deal,so...
Gradually Yahoo would be replaced by Bing on their sites so if your site is cached on Bing it would be shown on yahoo site too