Versatile weapons are one-handed, but creatures can use them two-handed. An attacker that does so gains a +1 bonus to the weapon’s damage rolls.
    A Small creature, such as a halfling, must use a versatile weapon two-handed and doesn’t gain the bonus to damage rolls.

Published in Player's Handbook, page(s) 217, Mordenkainen's Magnificent Emporium, page(s) 20, Rules Compendium, page(s) 270.