A day/night cycle would actually entail very little work and could be done fairly quickly. I think you're confusing day/night cycles with NPC schedules, which are two very different things.
Not in Larian's mind, last time Swen commented on it. He was quite clear that he had no intentions to bother with a day/night cycle that was simply aesthetic.
I always took the Day/Night issues as NPC schedules as well, which imo that is the biggest deal of the two. They have a light-sourced 3d game already, the actual visuals would be imo a lot less work than dealing with schedules. PoEt went Day/Night visuals, with static NPC's for example.
