yep it would be fairly simple to do. The problem with this method is IE6 does not support it and if you are doing client work they are not going to like the fact that it does not work on IE6.
Well, I meant that it would be cool if it was integrated with your browser so all links on all web sites would display like that. I never got in to user css files, so I'm not sure if that is something possible or not.
In fact, I just tried it and it does work with user CSS stylesheets. Just copy the CSS to a file, change the URLs for the images to an absolute URL on the web (possibly local?), and set your browser to use that CSS file.
However, it doesn't work well for every site, sometimes the icon is placed behind the link text.