Bear in mind that Spring Attack only protects you from AoOs incurred for moving... you can still provoke an AoO with your unarmed strike (unless you're a monk, have the IUS feat, wear gauntlets, have natural weapons, are casting a touch spell, are attacking an unarmed opponent, etc, etc, etc).
spring attack says you can only use it with a melee weapon, and while a natural weapon can be used in melee, it doesn't necessarily mean the same thing... that's why i'm asking