Can I replace the default login screen for the Pantheon Greeter?

Yes, you only need to Install pantheon-greeter and change the LightDM conf file like so:

• To get pantheon-greeter you'll need to add this PPA, open a terminal and type:

  sudo add-apt-repository ppa:elementary-os/daily && sudo apt-get update && sudo apt-get install pantheon-greeter elementary-theme fonts-open-sans-elementary fonts-raleway-elementary
  

• Then open gedit as root, type gksu gedit /etc/lightdm/lightdm.conf
• Change the line greeter-session= to look like this greeter-session=pantheon-greeter
• Log out