I just accidentally found my way to Super cool video hack: DIY Olympics virtual lane markers page that looked interesting. It says that a student, Xiaoyang Kao, recently figured out how to make his own Olympics-style lane markers.
The virtual lane graphics for swimming with css3 page tells how this video effect was done. Instead of an expensice special video effects software they used CSS3 and the Chrome engine to get the video graphics done. This is really impressive application for CSS3. CSS3 has an amazing property called perspective that can transform the graphics in the way needed in this application.
The graphics was mixed with the camera signal using a video mixer using subtractive keying system (removing the green).
You can try the graphics on the demo page if you have up to date browser that support the needed CSS3 features (I have tested the demo on newest Firefox and Chrome).