Login Logo is a plugin by markjaquith that allows you to customize the logo on the WordPress login screen. There is zero configuration. You just drop the logo file into your WordPress content directory, named login-logo.png and this plugin takes over.
The author says: that you should use a transparent background on the PNG image and keep the width below 326 pixels for best results. This plugin also works in the mu-plugins directory
Download and info: http://wordpress.org/extend/plugins/login-logo/