Looking at the 'big picture' we can see that there's a fair amount of potential conflict in such a merger between these two behemoths - however, there's also much that MS can gain from a takeover. Yahoo! is a strong brand, but has played second fiddle to google for far too long, in the UK at least.Their attempt at partnering with BT to increase their UK market share didn't have the desired effect, and Yahoo! are clearly struggling against Google who are even more on the ascendency.
I think there is a clear difference in philosophy between the two "Internet Portals" as they were once known. Google have always been open, and allowed the user to view the 'net' as it was. Their homepage historically has always been very 'empty' (before the days of iGoogle) - whereas Yahoo! wanted to frame the net and view it within a Yahoo! environment, so to speak. This has ultimately backfired, with the generic 'net' far more creative and awesome than any 'branded' net could be.In such a way, the relative paths of Yahoo! and MS appear to go down the same path. Providing their customers with a quality service - as long as you're a customer you'll reap the benefits. Whereas Google have gone for a more "laissez-faire" approach. They contribute (but don't control) a number of software projects. They donate their time and expertise in the way of the "Google Summer of Code" - which many companies and individuals have benefitted from - yet they can still wrap-up and package competitive SaaS software.
Moving to a different point, the main worry of this merger (should it go ahead) is what would happen to Zimbra. It was recently acquired by Yahoo! possibly to fortify their ageing web-mail interface. As one of the most prominent exchange-challenging mail server suites - I'm quite worried that an MS buyout could see that project severely disabled in the name of "killing competition." If so it would mark Microsoft's first attempt at being able to knock open source development... we shall wait and see.