There is a way to make a One Step DVD from a movie already on your hard drive. The option isn’t in the main menu of iDVD, but if you start iDVD and then look in the File menu, you’ll see the option “One Step from Movie…” That’s what you want.
Select that, and then navigate to your movie file on your hard drive. Put a blank DVD in your drive and iDVD will burn it. Your movie has to be in .mov, .mv4 or .dv format. As you know, One Step DVDs don’t have menus, so your movie will play as soon as you put it in a DVD player.