Anyone know of an enchantment for armor- not a material like mithril- that reduces it's arcane spell failure? If not, do you think there is a reason? What would it's + value be, if it could be computed that way?
BOEMII is definitely one of my favorites (BOEMI was pretty good too).
If you don't like the BOEM solution, though:
Making the armor out of mithril reduces arcane failure by 10% (DMG).
Bastion's Arms and Armor also has a spellcasting quality - this one reduces failure by 10% (+1 bonus). There's also the weightless quality that does the same, with some other benefits (reduces armor check by 5, raises max dex by 4) for a +2 bonus.