We've played with the beta for some time, but now the new MSN Search replaces the old Yahoo search engine which Microsoft have been using for a while now.
Quote Originally Posted by Bill Gates
I would like to introduce you to the new MSN Search service.

Based on your feedback, we’ve made MSN Search more precise and powerful, so you can quickly find the information you’re looking for.

Our mission at Microsoft is to use the power of software to solve our customers’ toughest problems. Searching the Internet today is a challenge, and it is estimated that nearly half of customers’ complex questions go unanswered. That’s why we’re proud of our new MSN Search service, a simple and powerful tool that helps you find the answers you want from sources as diverse as Web pages, images, news headlines, and Encarta.

Input from millions of our customers – including myself – was crucial to our efforts to make MSN Search the best it can be. If you have not already tried it, I encourage you to visit http://www.msn.co.uk/search now. We’d love to hear what you think, and I promise that we will continue to improve MSN based on your feedback.
One source suggests that Yahoo's search technology is still being used for the sponsored links.

Any chance of MSN Search displacing Google?