With the release of GDM 2 (2.22+), we lost the ability to theme (well, it's still possible but only to a small extent) the GDM login screen but with the arrival of Gnome Shell it should be very easy to theme everything Gnome Shell can be "skinned" with some basic CSS knowledge.
Browsing the Gnome Shell wiki @ live.gnome.org, I stumbled upon some great login screen and lock screen mockups which you can view below:
Login screen - select user:
Login screen - sign in:
Please note: these are only mockups so far.
What do you think about these login / lock screen mockups?
See also (Gnome Shell mockups):
- Gnome Shell Might Add Real Multiple Desktops
- Possible New GNOME 3 System Status Area Changes [Mockups]
[credits for the mockups: William Jon McCann @ live.gnome.org]