The situation I was thinking of was along the lines of:
You are Austria in game 1 and in game 2 you have a choice between ally with the German from game 1 or attacking him. You are worried that if you attack the other player in game 2 he may "forget" to bounce Russia out of Sweden in game 1.

In my opinion, any kind of consideration between current games is, by definition, metagaming. Using a player's history to make decisions is not metagaming, it's just good intel.
If you are afraid of attacking player X in game 1 because he might stab you in game 2, then that is pretty much exactly the same thing as saying to him, "If you don't attack me in game 1, then I won't attack you in game 2", which IS metagaming.
Each game should be taken on its own merits. You gotta do what is right for yourself in each game according to the circumstances and situation.
If you are playing with someone who WILL retalilate in game 2 for what you do in game 1, then that is not an honorable player, and games with that person should be avoided.
But, like I said, if you know that player X stabs his "ally" in every game as soon as there is even one center left undefended, that's not metagaming. That's just good intel.

So it seems like the question is, is it metagaming to be concerned that another player might metagame against you? That's quite the slippery slope with no easy answer.