Metagaming is an interesting thing and I know my personal interpretation of the definition and attitude about it continues to evolve over time. I do my best to ignore all past experiences with all players, even though it is pretty much impossible, and then I just talk a lot with everyone to get a sense of that person within that game. The most ruthless types often have a "tell" that I can pick up on, at least eventually. I do like anon games because I stink at figuring out who is who, so now I don't even try and make part of the challenge trying to figure out exactly how trustworthy everyone is merely based on in game actions and press.
As far as revenge goes, I'm a firm believer in getting revenge within a specific game, but after that game I will do my best to ignore recent stabs. Some people are extra nasty or however you want to describe them, so I'll do my best to not play with that person anymore because I've learned with a few people that it can be a personality thing and will never go away.
Anyway, I think this is all good to think about, there is a lot of grey area out there and we all have our line in the sand drawn in different places, but I suppose that's part of what makes games so interesting.