I recently discovered, after trying to change Safari's icon, that Apple's apps in Snow Leopard have read-only permissions enabled for all users except root:wheel. This eliminates users' ability to use copy-paste replacement icons in the Get Info panel.
As a workaround I quit the Finder, and relaunched it as root in Terminal:
sudo /System/Library/CoreServices/Finder.app/Contents/MacOS/Finder
I was then able to paste a replacement icon into Safari's Get Info panel. Then, after relaunching the Dock, my icon appeared.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090901205400914