David Edmundson, a KDE developer, has written a Qt library for making simple greeter engines. "I've written a Qt library for making greeter engines, as well as a very basic demo greeter which is more for testing than a real demo of what can be done. This library is designed to be very QML-ready, with a strong emphasis on using models rather than simple lists." LightDM is meant to be extended to handle various interfaces from Qt or GTK to having HTML/CSS-driven interfaces for this promising log-in manager.
Additionally, David had mentioned, "I've got an idea in my head for the direction I want to take it with QML themes that will be hopefully lead to the same broad range of designs that KDM has, but more flexible whilst keeping a really sensible config dialog."
Another KDE developer, Alex Fiestas, has been working on improvements in KDE SC 4.7 so that suspend / shutdown / user-switching can be done from the KDE desktop while using LightDM.
More details can be found in this blog post. Beyond allowing multiple interfaces/greeters to be written on-top and delivering fast performance, LightDM also prides itself on having low code complexity and support for plug-ins. Additional details can be found on its FreeDesktop.org page.