LightDM is set to replace GDM (the GNOME Display Manager) as the default log-in manager. LightDM is also likely to be used in Kubuntu instead of KDM, Lubuntu with LightDM rather than LXDM, etc.
LightDM offers the same fundamental features as the KDM/GDM managers, but it's code-base is dramatically smaller, it's easier to maintain / simpler to work on, and has full API documentation to make it easier to design themes / front-ends. It's also said to be more flexible than GDM.
LightDM also features a WebKit front-end for designing a log-in theme in HTML, for those who desire. There's also GTK and Qt front-ends, among other tool-kits, to cater towards your particular desktop. LightDM over Wayland rather than X.Org is also to be explored soon.
The switch from GDM to LightDM will occur soon to allow for greatest testing of LightDM during the development cycle of Ubuntu 11.10. Canonical has devoted one engineer to work on LightDM for the Oneiric cycle. Canonical is also pushing for LightDM to be adopted by the GNOME development community.
More details regarding the Light Display Manager can be found on its FreeDesktop.org page or on Launchpad. The UDS Budapest notes are here.