How can I take a screenshot of the login screen in Mac OS X?

"Try this: Grab 'impossible' screenshots

OS X includes a nice command-line screen capture utility named, simply enough, screencapture. While there’s not a ton of documentation on this program, man screencapture will give you the basic options. Once you’ve used ssh to connect to the Mac whose screen you wish to capture, you need to execute the screencapture command with root privileges. You can also specify some of the command line arguments, as shown in the man page. For instance, here’s how I captured the login window:

First, I had to get the login window on-screen. To do that, I just chose Login Window from the Fast User Switching menu in the menubar. If you haven’t enabled Fast User Switching yet, you’ll have to do so first—it’s in the Login Options section of the Accounts System Preferences panel.

On the second Mac, I opened Terminal (in Applications: Utilities) and used the command ssh to connect to the first. (See the No Files Left Behind article for more detailed info on using ssh to remotely access another Mac.)

On the second Mac, I then issued these commands:
$ cd ~/Desktop
$ sudo screencapture -ttiff loginwindow.tiff

The first line just switches me to the Desktop folder of the user that I logged into via ssh. The second line actually takes the screenshot; I’ve used -ttiff to set the type to TIFF ( -t is the ‘set type’ option, and tiff is the type, with no added spaces!), and then specified the name for the file. If you don’t want to hear the camera-clicking sound on the remote Mac, use the -x option."

You can use Grab by using the 10-second delay option, then putting the computer to sleep (with it set to show the login screen immediately upon sleep).