Recently i've tried that new display manager on a virtual machine. There is a nice wiki entry at wiki.gentoo.org which helped me installing lightdm. Unfortunately i failed which was because lightdm always started the gtk+ version.
When i forced the kde version, lightdm crashed. That was because of that issue.
After all, at the wiki there is a important configuration possibility missing. It's not shown how to change the greeter in the configuration file.
That is done with following statement in lightdm.conf:
lightdm-kde-greeter would start the qt/kde greeter, while lightdm-gtk-greeter would start the gtk version. For both you have to download the corresponding packages first. I mentioned that at the irc channel and i hope they add that.