Intresting technique, though I might not agree with him unless I try myself. It does save few server roundtrips which are wasted to download images. Read further here.
Yeah I have seen a similar technique used before to prevent lag in css based button hovers. I even wrote a blog post about it on one of my blogs:
http://websitetheme.com/archives/5-CSS-button-hovers-without-the-lag.html