Target creature gets +3/+0 and gains first strike until end of turn. (It deals combat damage before creatures without first strike.)
It's rather embarrassing to end up with your head mounted over his hearth.
Official Rulings for Slaughter Cry
10/1/2009 : If a creature doesn't have first strike, granting it first strike after combat damage has been dealt in the first combat damage step won't prevent it from dealing combat damage. It will still assign and deal its combat damage in the second combat damage step.