Armor check penalty and arcane spell failure are two separate stats for armor. Reducing the armor check penalty by making the armor masterwork has no effect on arcane spell failure. Using a special material, like mithril, can reduce both, but again, they're two separate functions.
As everybody knows, masterwork armors have a reduced armor check penalty, and armor check penalties have a direct relationship with the chance for arcane spell failure (i.e. they increase or decrease together). So, although my PHB doesn't say so, I'm wondering if masterwork armors also reduce the chance for arcane spell failure, and if not, why? Thanks!
As Kaisius says, there is no direct relationship between Arcane Spell Failure and Armour Check Penalties. They don't move in lockstep and hence one can be effected while the other is not. Masterwork armour is an example of this as is the bards ability to reduce the chance of Arcane Spell Failure when wearing some heavier armour even though the Armour Check Penalties remain the same.