As per the Rules Compendium at p.14, beating a Listen DC by 20 allows you to pinpoint the 5' square that the sound is coming from, a very important function when facing stealthy or invisible opponents.
The Keen-Eared Scout feat, Players Handbook 2 at p. 80, states that featholders can "determine the precise, current position of the creature or object that caused the sound" if they beat the DC by 10 or more. But it also states that featholders gain a +5 on Listen checks "to pinpoint invisible creatures." I'm not sure how to interpret these provisions. Does the +5 to pinpoint "stack" with the reduced threshold to locate position, effectively allowing you to pinpoint the invisible if you beat the Listen DC by 5?
Say you have Alertness and Keen-Eared Scout, and a normal Listen modifier of +5 (counting only your skill and Wisdom modifier plus the Alertness bonus but not any bonuses from Keen-Eared Scout). There is an invisible spellcaster casting a spell with a verbal component within 10 feet of you. That's a base Listen DC of 0. What is the lowest you would have to roll on the d20 to pinpoint which square the caster is in?