I'm not sure what you're talking about - dragging files onto icons always worked for me. What was maddening for me though in Leopard (10.5) is that you couldn't drag a file to an already-open application, like you could in the Windows taskbar (like to drag a file to a specific explorer window, for example).

Thankfully, you can do this now in 10.6 - drag the file to an already open application and hold it there for a few seconds. Then the app comes up, then (without ever releasing the mouse button) drag the file to the application window, and hold for a few seconds. Now you can drop your file in that window. The whole process takes about 5-6 seconds (which is agonizingly slow), and, IMO one of the worst aspects of application-driven window management (versus window-driven window management).