That works for great desktops but with a laptop I think you have to have the power cord plugged in with either the clamshell open (but still sleep) or a secondary monitor attached.
By the way, I tested this with my MBP 17" (mid 2009 model), and you are right: If I have the clamshell closed, apple TV will NOT wake up the computer (unless I were to attach a second monitor, as you suggest). Not an issue for me since I leave my laptop open all the time and basically use it as the family desktop. But if someone just has to leave their laptop clamshell closed, then yes there might be a problem.