[thejeff]

Actually it might be hard. Does "Friendly Fire" include spells? Just as easily justifiable. Much larger game effects.

Currently the same targeting code is used for arrows and single target missile type spells. They just do damage to whatever happens to be in the target square. I suppose you could check who owns the unit and who owns the attacker and what type of attack it is...
But honestly, it's way down on the list of most requested "simple" changes. And it's not going to happen.