How to power "arduino" from wall?
You should be VERY careful when working with line voltage in close proximity to low voltage electronics. It's easy to make a mistake and fry a circuit, or worse.
That being said, the cheapest and most realistic way to do this would be to run the Micro controller from a separate wall wort, and use a large relay to control the flow of AC current. There are plenty of wall converters that you can take apart and miniaturize enough to fit inside a wall, eg. an apple iPhone charger. The relay would then be directly in line with the switch, as in the schematic. When the switch is off, the microcontroller has no effect. When the power is on, the microcontroller will have control of the current the same way the light switch does, without shutting itself off.

This is just a very high-level overview, but should serve as a good place to start.