If you're tweaking your pages for high rankings in Google and Yahoo you now need to code for the MSN search algorithm as well.  MSN search used to use the results from other search engines, but they now have their own.  From what I've seen, it's very difficult to have the same page at the top of all three engines -- their ranking methods are so different.  You may want to think about creating three different sites, each with their own unique IP address (not on the same server), each with their own unique content related to your product or service, and do NOT link them together.  Others have had success getting good listings in all three search engines using this technique.  Of course, your mileage may vary...