[Gimli]

Every type of armor has an "arcane spell faliure %" associated with it, whatever that is is the percent chance a spell would fail when wearing that armor. But the choice to cast the spell would not be greyed out, you could still try to cast the spell, even in full plate mail. There's also the armored arcana feat that lets you reduce armor's spell failure percentage, I think it goes down by 5% per time you take the feat.