I have no credit for this GDM theme!
All I've done was change the background picture from the GDM theme “login only” (http://www.gnome-look.org/content/show.php/Login+Only?content=99683) to a photo with the carachters of Chuck tv series, which is broadcast on NBC.
Futhermore, this is my first work, so please leave a comment xD
By the way, I don't know if there are some writting mistakes, so sorry for my english =)
Bigger screenshot in http://img120.imageshack.us/img120/379/screenshotcopy.png