(For those not familiar with the LightDM display manager, read this posting.)
David Edmundson, the developer behind KDE packages such as its daemon manager, KIO, KWakeOnLAN, and the KDE Service Menu Editor, has recently been working on improving the KDE exprience with LightDM, as an alternative to KDE's KDM manager.
Edmundson has been working on Qt bindings for LightDM to allow for new Qt-LightDM front-ends, but he's also been working on a de facto KDE front-end. His KDE front-end for LightDM is capable of loading any QML file for providing different themes. With his design, LightDM themes can load Plasma widgets to this display manager or anything else that QML can handle.
On David Edmundson's blog he has written about his KDE-LightDM work along with providing screenshots and an Ubuntu PPA for those wishing to try out his new front-end and bindings.