So, I was at war with another empire, but they had just sent a huge army that I didn't have the resources to stop right now.  So I tried buying them off and getting them to sign a treaty with me.  The treaty screen said I could either get a peace treaty or a non-aggression pact.  I wasn't sure what the difference was, but the non-aggression pact said that we would agree not to attack each other for the duration of the pact.  Great, I'll take one of those!  Only when I hit "End Turn", he attacked me anyway.  Huh?

My best guess is that non-aggression pacts prevent you from declaring war, but don't have any effect if you're already at war.  This is EXTREMELY unintuitive.  If this is the intended behavior, it needs to be spelled out a lot more clearly, or the pact should only be available if you aren't at war.

Also, just as a general note, usually in these games "peace treaty" is the *stronger* diplomatic condition (the one that's hard to break) and "non-aggression pact" or "cease-fire" is the *weaker* one.  I was confused the first time I tried to backstab someone that I had a non-aggression pact with and discovered I wasn't allowed to.