The way Civ4 was developped (less units, MP-friendly, better AI, attack/defense becomes strength...) makes this idea more appealing. I've also tried the Diplomacy video game demo tonight. Not much to do there (the demo), but then you realize you have a video game where the AI can play Diplomacy with you.
Another sign that this idea is interesting, I think.
The next question is whether such a mod could be written for Civ4. My guess is NO, but then I don't know why I'm talking about (Civ4's possible moddibility).