My thought is that once a character's fumble die from Armor is higher than 1d8, the shield ceases to have any negative effect. Thus, a lightly armored character is penalized MORE for carrying a shield than a heavily armored character, which just seems odd.
I've decided that instead, I'm going to try using "+1d" for the fumble penalty for using a shield.
I'd like to hear your thoughts on this.
PS: Oh, and I'll waive this for Dwarves