To expand Samuel's comment into an answer, if you use the normal mechanical wall-switch and make sure no wires get stuck anywhere you solve two possible issues:
- You have a nice standard-looking switch
- You can use dirty tricks to get your low voltage, because the switch already makes it safe.
What do I mean? Well, Samuel said, if I'm not wrong, do something like this:
simulate this circuit – Schematic created using CircuitLab
In the modified situation, when it is as drawn, the light is off. When you flip the relay the light turns on. When you then flip the switch, it again interrupts the path.
If you make that relay a Latching Relay, such as these: EE2-5SNU NEC Relays you can change their connection with just a single pulse, which will not take much power if you keep the pulse short.
BEAR IN MIND! These are just an example I thought of, they are low current and not very high voltage (may do for light lamps or such, but no big halogen or high power LED lamps or Fluo's). But just to give you an idea.
You can then use a small non-isolated power supply, such as a dirty capacitor divider (they aren't allowed any more in production stuff, AFAIK) to get your low voltage.
For more insights into power supply decisions and why some are better or worse, depending on the application, this Stack answer is a pretty decent one:
http://electronics.stackexchange.com/a/41944/53769
Because you use a Mains-Rated AC wall switch, you are allowed to power your electronics with any non-isolated solution, because the switch and the wireless module will take care of protecting you, and the MCU, Relay and Zigbee module do not care if they are "unsafely connected", because that only relates to you getting zapped or not.
Just during testing/experimenting you would need to be extra careful not to zap yourself. If you are worried that might be one step too far, you might want to use a regulated fully isolated adapter until you're almost done tinkering. Just for your own safety.