If you're like me, you keep both the Applications folder and the Utilities folder as items in the Dock in order to access all your applications quickly. Howver, the Utilities folder has a generic folder icon so it's rather unintuitive to have it in the Dock since it's not clearly differentiated.
Unfortunately, the Utilities folder is a special folder and cannot have a custom icon. It's reset to the standard folder icon every time you log in. The way around this is to make an alias of the Utlities folder, paste a custom folder icon on the alias and drag it to the Dock. This way, you get your Utilities folder in the Dock with an icon of your choice. You'll need to keep the alias someplace, though, since deleting it will make the Dock item cease to function as a shortcut to the Utlities folder.
[robg adds: Instead of using an alias, I tried changing the ownership on Utilities to myself, but still had no luck with a custom icon. I'm sure there's a way to change the original icon -- logging in as root, perhaps? Anyone?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030417234333135