[Sundraoi]

What about weapons with poison properties already established on them?

Such as drow shortswords and that lot.

Thayan wrote, "The changes will apply to monster bolt, cone, and pulse poison attacks as well as PC/NPC poisoned weapons, food, and spells."

Since NPCs don't use vials of poison to poison their weapons, I would have to say, Yes. It will effect drow shortswords and the lot.