Ben of Binary Moon has taken the custom WordPress login a step further as one of his series of WordPress Tricks and Tips.
He created a plugin as well as an image template for the custom WordPress login.
The plugin itself sits in the plugins folder alongside the images – which you can change as required. The footer image has been changed to a gif with a transparent middle which means you can use any images you like for the main background and they will fit seamlessly.
Because this is a plugin and the images are kept separately from the admin folder upgrades a nice and easy. Just upgrade as normal. This is particularly handy for client sites where they may not be aware of what you have done to customise things.
Check out this and other tricks and tips over on Binary Moon.