Claudio @ ubuntued.info has created a tool for tweaking the LightDM appearance called Simple LightDM Manager.
For now, the tool lets you change the LightDM background (and select between an image or solid color) or the logo and that's about it. For now there aren't too many things you can tweak about LightDM anyway but hopefully when we'll have enough LighDM themes, this tool will also be able to switch between themes.
Here is a Simple LightDM Manager video recorded by Claudio:
To installl Simple LightDM Manager in Ubuntu 11.10 Oneiric Ocelot, use the following commands:
sudo apt-add-repository ppa:claudiocn/slm sudo apt-get update sudo apt-get install simple-lightdm-manager
Or you can manually download the package from the Simple LightDM Manager PPA HERE.
Thanks to Claudio for the tip; via ubuntued.info