I dont know if you have seen that, I have read about it before, but now that I used that I liked it a lot. Its all about scrolling of windows that are in the background, inactive. Imagine the following situation: You have MSN, Skype or whatever chat program, and on the same time you want to read an article in Safari, so you keep the MSN window active, keep chatting but at the same time you can scroll through the article in Safari without making the Safari window active. I really believe that Leopard is a triumph of customer focused engineering. No revolutionary features, but very well thought smaller features that at the end change the way we use our computers.