Snarf allows you to drag an ICNS file either onto its main window or onto its dock icon and will instantly update to use the dragged file as its new icon.
A friend of mine asked me today if I knew of a quick and easy way to preview an ICNS file as it would look on an application in Mac OS X's dock. I didn't, aside from opening an existing app's bundle and overwriting its icon or something like that.
Late tonight I decided that I could probably whip up a tiny app to accomplish this sort of thing in far less than an hour, and I did! So I give you: Snarf (requires Tiger or Leopard).