As of the release of Patch 0.7.9, poison damage-over-time effects appear to deal only 1 damage per stack to major bosses, namely the bosses in Ruined Coast and Titan’s Canyon (can’t remember their names, and can’t find information about them). Poison damages mobs normally, but deals next to zero damage to these enemies.
Discovered using the skill Serpent’s Strike on a level 39 Beastmaster wielding Draalsting. Screenshots of skill trees / gear / etc. available upon request.