That actually sounds like a good way to resolve the "should magic-users be allowed to wear armor?" dispute. Try this rule on for size:
Magic-Users can wear armor, although it is always treated as mundane unless they are also Fighters. When casting a spell, however, they are treated as wearing no armor, since they are too busy focusing on the spell to dodge or otherwise defend effectively. If they choose to use any defense against an attack, the spell is automatically interrupted and lost.That's a little easier to remember and handle than the spell failure house rule I was using before. Note that I am keeping the rule that magic armor is only magic for Fighters and Clerics.