Good news! The same people who gave us PowerSwitch Tail have produced a kit for a similar device to switch 240vAC devices on/off.
A more extended discussion of controlling household voltages...
Forgive my cross post to one other forum? As a former fire fighter, I have something of a bee in my bonnet about people using household voltages...
For an Arduino-specific How-To on controlling power, see:
http://arduino-info.wikispaces.com/ArduinoPower
And relays of the type pointed to in the article can be found here: http://goo.gl/8ZEQ8
Disclaimer: Some of this stuff is from my own shop...
Regards, Terry King
...In The Woods In Vermont
terry@yourduino.com
Well they look a bit expensive for what they are (the power switch tail stuff) - despite that it's pretty neat.
I will have a use for controlling AC 240V later in the year so I'm vaguely interested in this at the moment 
A little expensive compared to the cost of the components to "roll your own"... IF you have the skill and experience to roll your own SAFELY.
At under $20 to "do it right", I'm prepared to pay a premium to PowerSwitch for the work they've done "inventing" this "wheel", pulling together the bits needed, and making sure that things are safe.