There's another interesting little rule buried in Monsters & Treasure. The bonus from magic armor is supposed to be subtracted from the attacker's hit dice. Now, most people assume this is a subtraction from the d20 roll, or an equivalent adjustment of the armor class. But that's not what the original rules are saying at all; a monster's attack capability is based on its hit dice, and you are supposed to reduce this if a monster attacks someone in magic armor. In many cases, this is a leftward shift on the Monster Attacking table, but a 6 HD creature, for example, has the same chance of hitting an opponent in +1 armor as it does against an opponent in mundane armor. Really beefy monsters -- 14 HD or more -- aren't phased by magic armor in the slightest.
Of course, I've been using Target 20, so I haven't noticed this effect before. I'm wondering if it's worth it to try to implement the same effect outside the table.