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SRD, Magic Items II, Defending weapon wrote:

Defending: A defending weapon allows the wielder to transfer some or all of the sword’s enhancement bonus to his AC as a bonus that stacks with all others. As a free action, the wielder chooses how to allocate the weapon’s enhancement bonus at the start of his turn before using the weapon, and the effect to AC lasts until his next turn.

The defending weapon works like combat expertise - you can't gain the benefit without making an attack. And in this case - it is specific that you must be using the weapon.

Although not an official resource, this item in the 3.0 FAQ, confirms it:

The defending weapon works like combat expertise - you can't gain the benefit without making an attack. And in this case - it is specific that you must be using the weapon.

Although not an official resource, this item in the 3.0 FAQ, confirms it:

3.0 FAQ, pg27 wrote:

Do you have to actively wield a weapon of defending to use its power? Or could you hold a longsword of defending in your left hand, not use it to attack (so you are not actually using the two weapons) and still wield a sword in your right hand without penalties? Or is the defending bonus considered part of the normal parrying that happens in the background?
Using a weapon of defending works just like the Expertise feat. (You have to use an attack or full attack action.) You can’t use the weapon like a shield; if you hold the weapon in your off hand and claim an Armor Class bonus for it, you take all the penalties for fighting with two weapons, even if you don’t actually attack with the weapon.