The Google Talk Chatback badge I, and many other users, display on many websites does not work at the moment. When you try to use the badge it gives a Page Not Found (404) error.
Google know of this problem and work to provide a fix - for more info see Not Found Error 404 - when clicking on my 'chatback badge' in the Google Talk Help Forum.
It seems like that the first reports of this error appeared in March, so don't hold your breath.
The Chatback Badge now appears to be working once again.
No worky at the minute
Working once again