I completely disagree with this. The AI in a game should NOT be designed to play like that. The AI in a game should be designed to play to challenge you. To have an AI the way you describe would make anything past mid-game mind-numbing.
Do you find this to be the case in Civ4?
Civ4 has an AI that plays like a civilization, based on its diplomacy modifiers, not plays to win.
Its not a great AI for a number of reasons, but its one that is fun to play against.
Each leader has a AI parameters that determine the best relations they can have with another faction where they will still attack them, or be brought in a war against them.
Most AIs will *never* attack a player with whom it is friendly, some will never attack you if they are pleased.
Now, there is all kinds of scope for things like "you have a vast empire, -2 diplomacy modifier" or "you're building a spaceship, -2 diplomacy modifier". You can incorporate all those sorts of things into the existing system, that make it harder to maintain good relations.
[DuneWars mod has a terraforming victory that requires you to control and turn into grassland a % of the planet, as you get closer to this goal you get increasing diplomacy penalties with non-terraformers.]
I also don't mind a Master of Magic/FFH2 style "you are building the final wonder that wins you the game, I will attack you now!" mechanic for a final endgame war.
But for most of the game, diplomacy modifiers need to actually mean something. I need to know that I don't have to guard my border with my good friend in case he backstabs me and takes the cities in the heart of my empire. That's the whole payoff from diplomacy that keeps him a good friend; I can move those military units elsewhere.
Oh, and Aussie, please use paragraphs more. Its very hard to read those textblobs.