Guht- I like the elegance of your simple, get to the heart of it, approach... but I fear I side with the others in thinking...
a) it wouldn't really work very well in practice
b) would be expensive to implement
and you really DO need to build a BIG margin for safety into any work with 110v AC. "Little" booboos can easily kill. And that's kind of hard to "feel", so I'll add that they can also invalidate insurance policies or entail lawbreaking.
But! I still have similar projects running, and have fun with them!!
Split your energies and questions into these two parts...
1) What is the input to my system going to be?
2) What is the output to my system going to be?
To clarify that:
At the simplest level, the following "does" what you want to do... it at least has the skeleton!...
1) A push button switch, feeding an Arduino
2) An LED (and resistor) connected to the output of an Arduino
Bear with me! That REALLY is what you're talking about!
Eventually, you may get as far as directly switching a whole branch in your home, instead of the little LED... but whatever circuit switches the branch will, ultimately, trace back to the same output that you switch the LED with in the above.
Until you are sure you can do the first part, the input, in a way that meets your wishes, the second part is moot. And solving the first part isn't going to be as expensive. If there is no satisfactory solution to your wishes for what makes "things" happen, there's no need to solve the output side of the project.
A quick, easy, safe, reasonable-cost-for-what-it delivers answer to switching 110v is....http://powerswitchtail.com/default.aspx
(There's a thread about it at...http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1292089882/all
One last thing....
Rather than try to switch off whole branches, which will include the clock radio on the sideboard, and the answering machine in the den, try to identify the worst erg-eaters, and control them.
Heating and air conditioning are probably the biggest culprits, depending on where you live. And decent roof insulation is boring... but makes more difference than turning off a hundred TVs in a well designed standby.
I haven't started on all the fun and options for putting your "switch" and "LED" places other than next to the Arduino!! Those two "halves" deserve their own threads, and each can be subdivided many ways...