This partnership shows how similar the search results for all the search engines are. The search results are mattering less as they become more indistinguishable between each other. The partnership won't have much of an effect on SEO if you are already handling it properly and following the rules set by Google and the other search engines. Paid search may have some issues with this agreement as some AdWords campaigns may end up on Yahoo now. Google hasn't announced how they will handle the possible changes in costs and demographics yet.