I did a bunch of experimenting with 110V trying to see when voltage was applied. What I came up with is similar to the circuit you show except it uses the impedance of a capacitor. Here's the circuit, you can easily substitute the emitter part of an optoisolator for the led and get rid of the heat.
Of course, the cap has to be able to take the voltage. I was worried about the protective diode, but it works just fine. I have a couple of these as indicators for wall power to some devices, and they've been working great. I sort of went a bit nuts when I found out that they work so well.
The problem with only sensing voltage is things like hallway lights where you have several switches that can turn them on and off. I think I have a solution for that, but I'm waiting for parts to try it out.