Implementing a Microcontroller with an Iphone App to Control household lights?
There are several points that come to mind when thinking about the design for this project.      

Connecting Arduino to iPhone
There are 2 options for this use bluetooth or have a webserver running on a netduino, so it could be accessed from the web. Here are some tutorials on iPhone and Arduino.   

Mains/Line Power and Microcontroller
You are going to need some form of isolation between your control system and the lighting system because they both use different current(lighting if mains then is AC, microcontroller is DC) and different voltages(lighting 240v in the UK(120V in the states, while microcontroller uses ~5v). This could be done with a relay or opto-isolator. A relay is a device that is a mechanical switch which has an electro-magnet that pulls the contact instead of pressing it. Primarily used so you can interface different voltage systems. Such as your lighting system and Arduino.

Processing Software For Microcontroller
This is trivial basically when it receives a command turn an output on, it is also dependent on how you configure your software. 

Software For iPhone
This is the next thing you have to decide on. Remember iPhone development can only be done on a mac so maybe the webserver option is a better approach if you don't have a mac. Also could be accessed on any smartphone(not just iPhone). 

Overall I think your best approach is using an Arduino because you don't have to have an extensive electronics knowledge to get this project going, and the majority could be plugging a few wires in with this relay shield(just quickly googled that), then writing the software which there might be loads of examples around for what you are wanting todo. If I think of anything else I will add more.