I'm not a lawyer, but I did just read a book† written by a lawyer†† about this topic. I think it's possible that it can vary state to state, but in Delaware (a very popular place to form corporations) the law††† says:
The business and affairs of every corporation organized under this chapter shall be managed by or under the direction of a board of directors, except as may be otherwise provided in this chapter or in its certificate of incorporation.
The business and affairs of every corporation organized under this chapter shall be managed by or under the direction of a board of directors, except as may be otherwise provided in this chapter or in its certificate of incorporation.
† http://www.amazon.com/The-Shareholder-Value-Myth-Shareholder...
†† http://www.lawschool.cornell.edu/faculty/bio_lynn_stout.cfm
††† http://delcode.delaware.gov/title8/c001/sc04/index.shtml