I just noticed that in 10.5 you can scroll in any unfocused window by simply moving the cursor above it and start to scroll up/down with the mouse wheel. No need to pull the window into focus.

[robg adds: You could scroll background windows in 10.4, too, but you had to be able to see the scrollbar; Command-dragging would then scroll the window. In 10.5, as long as you can see some of the window, it will scroll when the mouse is within its visible area. This one is documented on Apple's 300 tips page, but I bet many (including myself) may have not even noticed it...]