Creating a custom login logo is a very simple task and will give the appearance of a customized content management system created just for your client.
Update: Using the method outlined below will work great, however, when you update wordpress you (or your client) will have to redo these steps. Using a plug-in like this one can solve the problem.
Create an image called logo-login.gif that is approximately 310px by 70px.
Use an ftp client to replace the current image located in the wp-admin > images folder.
Refresh your browser; You now have a customized login page perfect for impressing clients!
You can follow a similar protocol to replace the WordPress Dashboard logo; replace the wp-logo.gif image and make your image approximately 30px by 30px.