For some customers we don’t only change the frontend look&feel but also the backend of Magento. Once you have a customized version of the backend you would most probably want to change the favicon as well.
The default location for the admin favicon.ico is:
http://yourdomain.com/skin/adminhtml/default/default/favicon.ico
Replace the icon and don’t forget to refresh Magento’s cache.
If that shouldn’t help, go to the backend and view the page source. Right in the beginning of the file is a tag looking like
<link rel="icon" href="http://yourdomain.com/skin/adminhtml/default/default/favicon.ico" type="image/x-icon" />
The path inside the href shows you where the favicon is.
