Well, if you want to be reallly geeky, it is the acceleration due to Earth's Gravity at one specific altitude, which I rather suspect is at least ten miles from sea level - it'd be pretty weird if such a round number worked exactly at sea level.
This also does not take into account the tidal force exerted by the sun and (more particularly) the moon, or, for that matter, the fact that the Earth is not a perfect sphere.
nine9: They're different versions of the same figure. 9,821 m/s^2 and 32.something ft/s^2 are actually equivalent. | 
