If you fail on the disarm attempt, the defender may immediately react and attempt to disarm you with the same sort of opposed melee attack roll. His attempt does not provoke an attack of opportunity from you. If he fails his disarm attempt, you do not subsequently get a free disarm attempt against him.
kjenks said:And each Disarm attempt would normally provoke an AoO, but this is usually only important against a creature with the Combat Reflexes feat, since all others are limited to one AoO per round.
In addition, remember that failing a Disarm check gives your opponent a chance to Disarm you, making this tactic less useful.