A MSFT+YHOO merger smacks of desperation to me. Neither company has shown leadership in the search or advertising markets in years.
Considering the debacle that was the Vista release, it will be quite feat if a coherent web presence is the result of such a merger. YHOO alone suffers from a bi-polar disorder with services such as Yahoo! photos and Flickr competing against each other.
Combine that with Microsoft Live! (what ever that is) and you've got a Hydra monster of services and an operations nightmare to go along. And that's their strategy to stop Google. Good luck with that.
Update: Yahoo! photos is shutting down.