"Might" is the operative word. First, the deal has to be approved by the U.S. government and the EU. Then Microsoft/Yahoo will need to attract enough advertisers and publishers to reach critical mass--and that's tougher than it sounds, especially for contextual advertising programs like today's AdSense and YPN where inventory is needed for untold thousands of niches and subnichs.
It's also unlikely that Microsoft's goals are as simple as competing with Google for contextual text ads. Microsoft is almost certainly looking at the bigger picture, and especially at areas (such as display and "rich media" advertising) where Google isn't yet a major player.
In short, if you're hoping that a Microsoft/Yahoo deal will create a cornucopia of easy money for unhappy AdSense publishers, you're likely to be disappointed.