Yes, I always thought that. But if you've got UltraVNC running, it has a Ctrl-Alt-Delete button, which sends that keycode to the target computer. And it works. Which I thought was odd.
You can send the combo just fine by whatever means you choose, but since you can't hook your own apps into launching on receiving those keys, you know that the program that pops up is the real login screen, and not a fake...