maniac (me) also said in another thread 'I'm learning all the time - I never knew meta-gaming was frowned upon. That's it from now on i'm a clean sheet player.'

Bad and unavoidable, though luckily reducible.

Not if we could play anonomous games

Both posts above mine reflect my thoughts entirely.

Although it has to be considered that anonymous games will make multi-accounters hard to track down by our local vigilantes. I suppose you could publish username at the conclusion of the game.

This is definitely "frowned upon", BUT:

1) If this is a league game (?) then this is definitely to be expected, and is one of the more interesting factors coming into play, and
2) it's perfectly okay to make THREATS to metagame like this. This is a game of bluffs and lies, so really anything you can do to get another player to do what you want (threatening to metagame, go CD, spread slander about them, hunt them down and kill them or a loved one, etc.) is perfectly legal. Actually following through with it is a different matter.

Anonymous games wouldn't solve anything. they could email each other and tell people which game they're in. in fact, that would make it worse, because it would be harder to track them.

The things that people bring from one game to another that is more often seen is the knowledge that a player is a good or a bad player not that he is a good or bad ally. People that stab, lie whatever in a quality way that is respected usually have no ill effects. Those who lie foolishly or stab poorly or play in an irratic manner generally find the memories of their behavior more often held against them.
It is largely considered a major NO NO to make cross game alliances, threats or to bring feuds from one game to another. This is not only because the other party involved is affected but because your own play is greatly screwed since you become a captive of your past relations with the player thus reducing your flexibility and generally reducing the over all PLAY of your own game.
I would also say that from the start of the hobby the idea of bringing one game into another was just about the only real major NO NO other than the unthinkable sin of playing double positions
You see this very much in play in face to face games where experienced players with each other always try to avoid a reputation of always allying or always fighting or whatever. Predictability can be a very tricky thing to deal with.

xgongiveit2ya55 'threatening to hunt people down and kill them or their loved ones is not perfectly legal (in the UK at least) even though it doesn't break the rules of the game.

BOTH

I've always thought that what Edi said above was the best policy. You should bring knowledge of how well another player PLAYS with you across games, but never grudges or alliances. Honestly, if you do your play will suffer, and you really only hurt yourself.

To answer the first question though, I would say that it's more or less inevitable that people will do it, and though they shouldn't I don't think it's "against the rules." Really the key is just to identify these players and exploit this weakness.

I play so many games I can;t be bothered to keep track of WHO I'm playing... maybe you guys should just play more and relax a bit :)

Well, I have to admit I keep a blacklist :)
It includes people who I have found out to be both unreliable and not so good at the game (seeing the map balance).
To clarify, I am perfectly ok if someone stabs me in order to win, survive, or substantially improve his position.
I do have a problem with someone back-stabbing in cases when this does not make sense (though I agree that 'sensible' is subject to interpretation). Anyway, usually these would be people who attack you as a revenge or simply based on ego/pride (which happens a lot if you have a weaker ally).
So, I will admit to partial metagaming - but I am somehow ok with this 'flaw' :)