Until Apple gets around to building this functionality into the OS, I'm going to show you how to do it on your own.
Step 1: Set your desktop background image. Open up 'Desktop and Screen Saver' in System Preferences and select your desired desktop image.
Step 2: Change your login image. There are two ways to do this: the hard way and the Loginox way. I like to make things easy, so I'm going the Loginox way. Loginox, by Branox Software, is a simple little freeware application that lets you change your login image with drag and drop simplicity.
Download the app and open it up. You'll see a single window that shows your current (most likely, the default) login background. Next to that will be an empty window. Simply drag the image you used for your desktop background into the empty Loginox well, click 'Set new Login Image', enter your admin password and you're done.
The application requires your admin password because it is temporarily changing a system file on the root level of your computer. If you ever decide you want to go back to the default login image Apple provides, just open Loginox and click 'Reset to Default' and you're back to normal.
That's it. Two steps. You're done. Now logout and log back in. You'll see your new login image screen and, if you used the same image for your desktop background, you'll notice the smooth transition to your desktop after you log in.
Is there a use for this besides the slight cool factor? Sure. If you run a computer lab or a business with lots of different work stations, you can use Loginox to give each computer a unique login screen - perhaps identifying which computer has what apps on it or what the computer is used for.
Loginox has just been updated for Mac OS X Snow Leopard and requires 10.5 or above.
UPDATE: A lot of you have asked for the wallpaper I used. It's by a wonderful graphic artist named PoSmedley and can be found here.