Not only does the BM Custom Login plugin give you the ability to customize the generic WordPress login page, there are templates included in the download. A PSD for the main image, and a Fireworks PNG (vector) footer. I thought the instructions were a little confusing, but all I did was use the templates and saved my 2 images as ?login-bkg-tile.jpg? and ?login-bkg-bottom.gif?. Then I replaced those same two files in ~/wp-content/plugins/bm-custom-login/images, and it worked instantly. No need to mess with the WordPress core files at all!
Some Other Examples: