Andrew A. Schwartz (Wachtell, Lipton, Rosen & Katz, New York) has published The Patent Office Meets The Poison Pill: Why Legal Methods Cannot Be Patented, 20 Harv. J.L. & Tech. 333 (2007).  Here is the Conclusion:

“Invention,” as used in § 101 of the Patent Act, means anything made by man that utilizes or harnesses a law of nature for human benefit. Technological innovations, from the telephone to Tylenol, satisfy this definition, as do most “business methods.” But legal methods, such as the poison pill or a tax strategy, do not. Legal methods, no matter how novel or valuable, are not “inventions,” and are therefore not eligible for patenting.