That's exactly what I need to know (or need the arduino to know), just whether it's on or not, and that's a pretty sweet solution. I've got some of those neon lights with matching resistors kicking around already too.
BTW, I'll tell you all what this is for, maybe you can offer other advice about it, more brains are better. I live in the Northeast US, cold in the winter, oil fired boiler with big old radiators. I have three zones. I'd like to be able to track time and duration of the boiler firing and the zones' calls for heat so I can track my oil usage and find out which zone is burning the most oil at what time of day, etc, etc. So the 120VAC I want to detect are going to the circulator pumps (3 zones) plus the boiler thermostat that activates the big transformer. I already put my meter on it so I know just where to hook this up.