Wings of Redemption
In addition to all the normal rules, add the following paragraph to the Diehard feat:
A character with the Diehard feat does not become unconscious when his Nonlethal damage total exceeds his current HP. Instead, he becomes staggered when his nonlethal damage equals or exceeds his current HP by less than 10. When his nonlethal damage total exceeds his current HP by 10 or more, he falls unconscious.
This sounds rather random, but it is to (consistently) work around the following scenario: a character with diehard is acting at -1 HP. He can take up to 8 damage from, say, a sword, and aside from being closer to death, have no ill effects compared to when he had -1 HP. But even 1 point of nonlethal damage from an unarmed strike, whip, bolas, or sap would isntantly render him unconscious.
The extra wiggle room is just for consistency, and prevents a character with diehard and a max and current HP of 10 being more adversely affected by 12 points of nonlethal damage (or 6 nonlethal and 6 lethal) than if he had taken 12 points of lethal damage. Lethal damage hurts more, and so should always be worse for the character!