So I got my Google Chromecast today and after a bit of messing around I found this cool neat feature that allows you to mirror your entire display to the chromecast device.
This is how you do it.
1) In Google chrome browser on your computer click the Google cast extension and then click the little arrow in the extension and select "cast entire screen (experimental) I've attached a screenshot.
2) after completing step one select your chromecast device and walla you see your screen on your TV
This seems to be an experimental feature so let me know how it works for you, so far I've tested it on my Linux Mint 15 laptop and it works nicely there is a minor delay but at least its something