The MS-YHOO deal is keeping the merger arb guys up at night.

Michael Arrington notes that the Yahoo acquisition is getting very expensive, in terms of Microsoft’s market cap; Microsoft has “lost nearly $40 billion in market cap in the eight trading days since they made their offer.” In other words, “Microsoft has shrunk by a Yahoo in the last eight days.”

Henry Blodget has a must-read piece on the logic of the deal; he argues that Microsoft is confusing the ad-driven consumer business with the license-driven corporate business. This makes sense to me; Microsoft needs to defend the Office franchise in the corporate environment, where even Google is getting license revenue ($50/user/year for Google Apps) in lieu of advertising. Enterprises aren’t going to use ad-supported free software, at least not in any future I can see. So why should Microsoft take on the pain that is Yahoo for the consumer side? Blodget writes, “Put differently, the part of Google that threatens Microsoft’s core Windows and Office business is Google Apps, not Google Search.”