Earlier this month, Microsoft borrowed $2.25 billion in unsecured debt. What in the world possesses a company with $40 billion in cash and short-term securities to go out and borrow money?

Rock-bottom interest rates are one reason. But the bizarre, byzantine U.S. tax code seems to be another.

Microsoft declined to comment on whether its recent borrowing was partly driven by tax considerations. But, like many purportedly cash-rich companies, Microsoft can't bring home much of its cash without writing a fat check to the Internal Revenue Service.

Politicians have been carping about the more than $2 trillion in cash sitting idle in corporate coffers even as unemployment remains high. But much of that cash isn't in the U.S.; it is abroad. And it isn't likely to come back home unless U.S. tax laws change.

David Zion, a tax and accounting analyst at Credit Suisse , estimates that the companies in the Standard & Poor's 500-stock index have "north of $1 trillion" in undistributed foreign earnings, or profits that have been parked overseas to avoid U.S. tax. Not all of that is cash; some is in the form of inventories or other assets.