I've always disliked Safari's RSS icon (and the even-worse XML icon) used for RSS feeds, so as soon as Microsoft and Firefox agreed upon a new icon, I immediately wanted Safari to use it. It turns out to be pretty simple, and you can see the results in the image at right.
First, download the new standard icon from Matt Brett's new Feed Icons site. Second, use GraphicConverter or other means to save the 16x16 icons as .tif files. Finally, replace the ShowRSSButton.tif and ShowRSSButton_Pressed.tif files in /Applications -> Safari.app -> (Control-click and Show Package Contents) -> Contents -> Resources with the orange and gray converted files, respectively.
The usual warnings apply about making a copy before modifying files, etc.

