exactly, an optical isolator (basicly a photo-transistor and a led packed in a dip chip) would handle the signaling from high voltage to the low voltage arduino as long as you plop an appropriate resistor in line with the led side so it doesnt burn out, and if its ac just rectify it before hand
not super complicated (couple o parts), its just a matter of knowing what your dealing with, and what your planning to do
(and yes I do have a habit of making post's more complicated, but its about finding the best solution for your intended application)