X10, while cheap, is notoriously unreliable. This, however, depends on your home's wiring so your mileage may vary. If you decide to go the wireless route, you can get some pretty cheap wireless modules and pair them with standalone Atmel chips. As an example, my sensors all have an ATTiny84's (programmed using an ISP programmer and the Arduino IDE) paired with RFM12B Transceivers. The result costs less than $10 per node. They all run off batteries and send out and poll for periodic data, to a standalone Arduino (328P chip) which keeps track of all the sensors' data. It's also connected over I2C to a regular Arduino with an Ethernet shield so I can communicate with the Android.
Since you're not going to deal with wiring, I would recommend some sort of remote power switches that you can hack and control with your Arduino. I haven't used any, but I think others have successfully used an Arduino in conjunction with these:http://www.amazon.com/3-pack-Wireless-Remote-Control-Outlet/dp/B0064PKG3Q/ref=sr_1_3?ie=UTF8&qid=1338042555&sr=8-3
A good resource, if you do decide to go the wireless route is JeeLabs:http://jeelabs.net/
His libraries for the HopeRF transceivers are very powerful and easy to use.