Lots of blog provide a way of anonymous commenting. But if you want to copy your comment and  also post on your own blog? You can surely use trackback, but as a matter of fact, lots of blog providers do not implement trackback perfect as commenting and you may not like to open a new window/tab to write a comment (maybe copying trackback url or other things).

And then, what about writing a firefox plugin to hook the commenting and post to the comments back to the user' own blog?