Corporations as legal entities have always been able to perform commercial activities, similar to a person acting as a sole proprietor, such as entering into a contract or owning property. Therefore corporations have always had some limited amount of 'personhood', which has allowed corporations to conduct business while shielding individual stockholders from personal financial risk (i.e., protecting personal assets which were not invested in the corporation).

The stronger concept of corporate personhood is often traced to the 1886 U.S. Supreme Court case Santa Clara County v. Southern Pacific Railroad Company (118 U.S. 394), which provided some greater degree of protection from arbitrary state action. In their decision, the justices gave no explanation of how an amendment about the rights of former slaves should also apply to corporations.

Also see creature of statute.

http://en.wikipedia.org/wiki/Corporate_personhood