A new Unity Greeter (the default LightDM greeter for Ubuntu) version has been released for testing in the Ubuntu Desktop PPA, and it brings support for reading the user background, along with other minor tweaks.
That means that the login screen background should now match your desktop background. This also works if there are multiple users - in this case, the LightDM login screen background changes depending on which user you select.
Here's a video demoing the new Unity Greeter (in Ubuntu 12.04 Precise Pangolin):
The new Unity Greeter, along with a new LightDM version are only available for Ubuntu 12.04 Precise Pangolin users and can be installed by adding the Ubuntu Desktop PPA. The packages should also be uploaded to the official Precise repositories soon.