It's been posted before, but I like the idea. Maybe a whitelist for Multiplayer? (You whitelist people and they don't get zapped.)
Yea, this could get messy in MP, with your turrets Zapping friendlies.
Maybe a GUI were you could define behaviour (lethal, explosivle, non- lethal, economic, fast fire ,e.t.c) and a slot were you could add players for the turret NOT to zap and a couple of buttons to define mob-interaction (spare villagers, hit cows, and so on)
Nice idea, the Tesla is awesome as a secret trap, but a unusable in SMP since it will zap EVERYTHING. (i had a fight in smp, were my teslas prefered to kill birds other than Hostile players 
)
Already suggested (Upgradabl Tesla Coil - Laser Turret)
I guess denied. But I don't really remember.
At least, wait for Industrium-Conflict 
