werk said:
It would make sense to me that you would need to do lethal damage in order for the subject to bleed to death. You could kill a Xixecal with a +1 Gauntlet of Wounding without ever doing any damage (granted, it would take 40 hits).
Two bits of relevant text (well one, and a tangent), firstly from Damage Reduction:
"Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury type poison, a monk’s stunning, and injury type disease. Damage reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact."
So if you do no damage because damage reduction negated it, I'd expect that the wounding wouldn't kick in, as its clearly an injury type special effect. Now by the letter of the rules, this only applies if damage reduction negates it... this passage doesn't have any bearing on wounding subdual weapons, for example... but its probably not unreasonable to use it for the basis of ruling on that and similar matters.
Secondly, and more directly relevantly, from Regeneration:
"An attack that can cause instant death only threatens the creature with death if it is delivered by weapons that deal it lethal damage."
Now, you could take from that sentence that you can drain a troll down to 1 con with a wounding weapon, but can't kill it by dropping its con to 0... thats a bit of a convoluted reading tho.