Ok, first the suggestion, the issue has supplanted the AI being stupid as the thing pissing me off the most when I play single player.
Some manner of seeing who likes who. Civ4 style chart showing colored lines between them indicating war status would be more than sufficient. If they cheat like a bitch and randomly ally with each other right off the start, I'll be able to tolerate it, but not knowing that all three of my neighbors are allies and having an alliance with one of them really makes the alliance rather pointless. I might as well stick with a peace treaty and not have to worry about them declaring war on me when I drop to 70 favorability after two failed missions.
I've played through two large scale games now and they both broke down into a game where almost all the ai were allied with each other and the only two still at war were my own allies. I couldn't get them to do anything because they liked everyone but each other, bloody wonderful.
Now the question. How does the AI diplomacy work? Finding three AI defending the same planet five minutes in is rather... irritating. Do they have some sort of system for deciding things or do they just randomly request an alliance and go at it? I've seen an AI bribe my aggression on the guy he's preventing me from attacking for the first diplomacy event.