Can I screen-share with a computer that has no monitor?
Yes you can. Enable Screen Sharing by connecting the computer to a display (and keyboard) once. Then you can easily unplug display/keyboard and access the computer via Screen Sharing.
As long as you do not mess with the network and screen sharing settings of the headless computer you also can turn it off and reboot it without loosing the ability to screen share again.
Two things to be aware of:
The "virtual" screen on the headless Mac will be limited to only 4 standard resolutions (800x600, 1024x768, 1280x1024 and 1680x1050). All except the last one are 4:3 aspect ratio.
The "login" screen may be displayed in a completely different resolution than the working desktop. I haven't been able to determine how this screen size is determined, possibly it depends on the last resolution used when a standard monitor was connected.
It's a great way to set up a home media server. Good luck.
Yes, I do this very thing with a "headless" Mac. I encode video on a Pro, but send it to the MDD G4. I can then pull it to any TiVo in the house. In other words, the headless mac acts as a server. I can also see its (non existant) 'screen' through VNC.