Edit @ KarinsDad
I dont think it would be metagaming if you play it right... I had a fight where a group of lower level monsters all decided to rush the big shiny scary-looking human right out of the gate.
After a couple rounds of being wholly ineffective against this metal-clad behemoth (weapons just bouncing off the armor/easily parried) they decided they would be better served picking an easier target... like that guy in the robes!
I believe that ANYONE can tell if it's attacks are been ineffective.See that isn't metagaming. Creatures above animal intelligence should be able to tell they are having no effect upon a target and switching to a different one is perfectly reasonable.
I don't think it's a fallacy, not in the strict term at least.This is a common fallacy.
But note that they can't really force them to do that.Defenders should have the best AC they can get. They should then take powers like Come and Get It, and Feats like Daunting Challenge, and Mark as many foes as possible, and try to keep as many foes attacking them as possible.
Ok, now onto the meat of the argument.The foe should not know the AC of any PC. If they do, then the DM is metagaming.
Of course, a fighter who sacrifices defense for greater damage output is providing a more compelling reason to attack him, since he's easier to hit and more dangerous to ignore.
See that isn't metagaming. Creatures above animal intelligence should be able to tell they are having no effect upon a target and switching to a different one is perfectly reasonable. I think KarinsDad was saying it would be metagaming (and poor DMing in general) were the creatures to immediately ignore the defender (due to the DM knowing the AC) and attack other targets.
But note that they can't really force them to do that.
They can only force them to a Catch-22 deal: hit a highly durable foe, or suffer damage/conditions for hitting a softer target.
So a trade-off WILL be made.
And this is where an extremely high AC can result in a problem.
How do the monsters KNOW that the Fighter sacrificed defense for greater damage? The DM knows. But, the DM shouldn't necessarily be using that information to make the monsters decisions.
Dumb monsters should be played dumb.
Note: I'm not saying that monsters cannot figure out that cloth armor protects less than plate. But, there are many armors when combined with the protection of a Mark that are as hard to hit as plate (i.e. hide or better is typically that way) and most monsters should rarely think that the lighter armored foes are by definition easier to hit if the monster is marked.
But, if it is a nearly every encounter thing where the DM has the monsters "decide" to avoid the heavily armored Fighter, then the DM should reevaluate his priorities.
On the matter of who's softer and who's harder, I assume that the enemies know this at first glance. Obviously some exceptions exist, but I'm talking in general.
This is as fair as when you, as DM, describe the enemies and drop hints on which enemies are tougher and which are softer, which are mobile, ranged, etc.
I assume you DO tell the players this stuff.
To give a quick example: if you don't know that two enemies gain AC when adjacent, you'll never use your forced movement powers to aid a melee combatant that's attacking them.
It's a lot more fun and interesting if the opponents know of this feature.
I tell the defense score of a target when you have line of sight to the attacker and the target when an attack is made.
This applies to both sides.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.