New Design

GDM was rewritten during the GNOME 2.21 release cycle.

Motivations

Better fast-user-switching support (bug #343539 etc)
Enable a smarter people chooser in the greeter
Better ConsoleKit integration (seat awareness, coordination etc)
Facilitate creating a new blingier greeter
Dynamically configure displays
Allow session agents to run in the greeter session (gnome-power-manager etc)
Have the ability to only run a single greeter per seat (currently gdmflexiserver will start any number of them)
Use PolicyKit for reboot etc authorization/handling
Use a better configuration mechanism that is more compatible with a hypothetical systemwide D-Bus based GConf
Provide a D-Bus API so that agents like fast-user-switch applet can be written more easily and operate more efficiently
Fix all the horrible non-reentrant POSIX signal handling and various race conditions in the current code (bug #336549 etc)
Make it easier to do "hot desking" type things
Use a more modern design to simplify maintence and enhance flexibility (use of GObject etc)
Use a more robust, secure, and flexible IPC