The Collaborative International Dictionary of English v.0.48:

Bedpost \Bed"post`\, n.
   1. One of the four standards that support a bedstead or the
      canopy over a bedstead.
      [1913 Webster]

   2. Anciently, a post or pin on each side of the bed to keep
      the clothes from falling off. See Bedstaff. --Brewer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bedpost
    n 1: any of 4 vertical supports at the corners of a bedstead