Text (US+errata): Instead of the attacking player, you assign the combat damage of creatures attacking you. [Oracle 1999/11/01]
You follow all the normal rules for damage assignment. For example, you can assign more damage to a blocking creature than its toughness, and if a creature has Trample (see Rule 702.17) you follow the Trample rules as appropriate. [Urza's FAQ 1998/10/05]
Damage assignment choices from Trample (see Rule 702.17), Thorn Elemental, and other things which allow alternative damage assignments for attacking creatures are all made by you. [bethmo 1999/10/05]
