Wall of Text that is both on and off topic and contains a rant ahead:
The AI, as far as I can tell, only backstabs allies based on how many "allied victors" you set. For instance, if you have 4 people allied, and there can only be three victors - there will be a backstabbing. It will, however, be you 100% of the time. So, no, not really... the AI doesn't backstab, and it will betray you first despite relations.
You will have to be the one doing backstabbing - early and often - to assure that you come out ahead. In a game I had the other day, everyone was allied with everyone else... I just offed the other AI one at a time - no one would attack or defend their allies at all, it became a stagnant boring mess. My only option was to cheese it up (which I did), or just go to war with everyone (I didn't feel like tacking six 1500supply fleets, though).
In short, the diplomacy system, while better than what we used to have... still fails. It may have some potential in MP, but for SP it kind of sucks... I sometimes wonder if their focus on balancing it for the MP factor isn't what's holding it back in the SP factor.
Although frankly... I don't have any desire to really play it MP... I don't want to participate in a race to pacts/dip cruisers. It seems to me (in theory) that pacts/dip cruiser effects just make it even harder to be the odd man out in a game against humans. In Entrenchment... you were pretty well screwed if you got ganged up on, but it was still possible if you were just that much better/luckier than your enemies. With these extra bonuses from diplomacy? I don't know... it almost seems like a buffer of sorts...
Say previously one good player was fighting against two average players. My money would be on the good player. With Diplomacy however, those two can pact up and gain substantial bonuses that make it quite a bit more difficult on the good player. I do not like nor agree with anything that gives people a "handicap"... there should be no "artificial skill" in a game. I don't know if I'm making my opinion clear or not, so I'll just wrap this up.
I'm disappointed w/ the Diplomacy system. The diplomacy system is not realistic, the betrayals are 99% on the human player. And yes, you can bribe enemies with goods. Each race gets a buff to a certain type of donation that makes donating that material give a substantial boost to relations, and in time it can make even an enemy a friend.