The rules regarding the Two-Weapon Fighting feats in D&D 4th Edition seem pretty clear, but I'm still not sure how they apply to double weapons. The Adventurer's Vault states "wielding a double weapon is like wielding a weapon in each hand" and it states that you can use either end to attack, but there's nothing that states how Two-Weapon Fighting applies. Would Two-Weapon Fighting and its later feats apply to double weapons, or do the feats only work if the PC is wielding two separate weapons in each hand?
Wielding a double weapon is wielding two melee weapons (one in each hand) for all rules purposes.
It takes a feat to get the superior weapon proficiency you need to use a double weapon in the first place, and pretty much every power that cares about the damage of your off-hand weapon also has a requirement of "You must be wielding two melee weapons."
So "is like wielding a weapon in each hand" should be read as "consider these equivalent for rules purposes", rather than as an attempt to make some philosophical point.