The icon should be viewable at 512x512, 114x114, 72x72 and 57x57 pixel sizes. I'm happy to receive just one single image at 512x512 and scale it myself so long as it looks ok when scaled down.
I'd like to draw on the colors and feel of the digg.com website so that customers recognize the app as being designed for digg specifically. Please check http://www.digg.com to get a feel for their style.
I've considered the use of a shovel (spade) or earth mover in the icon but there is no requirement to use either of these things and I welcome any suggestions.
I also have an existing look and feel for the app (see screenshots attached) and if possible the icon should work alongside this.