The thing you're talking about will be implemented by ISP's and other providers of network connectivity. They could cause your connection to Yahoo to be slower than your connection to, say, Google if Google pays but Yahoo doesn't.
Regardless, no, this has not been implemented yet. It's just the Internet as usual and more specifically Yahoo has usual. (Yahoo's servers seem to have issues. I have no inside knowledge of this, but from personal experience, I have always had problems with Yahoo dating back to when it was about as popular as Webcrawler. What's that? Exactly.)
Anyway, Senator Ron Wyden has introduced legislation to prohibit this practice should it ever become viable, which it isn't at present. I doubt this legislation will go anywhere since it is distinctly against corporate interests, but its introduction indicates two things that are at least somewhat positive. First, it's not a done-deal yet, and two, some people who have influence over this sort of thing "get it" and realize it is a bad idea for consumers.
http://www.nytimes.com/2006/03/02/technology/02online.html?_r=3&pagewanted=all&oref=slogin&oref=slogin&oref=slogin