Note to Kofi Annan and many other commentators: to quote the philosopher Inigo Montoya, "You keep using that word. I do not think it means what you think it means."
"Proportionate response" in Just War theory does not mean that if they kill 10 of your guys, you only "get" to kill 10 of theirs. That would be a return to the old "law of the Talion," an eye for an eye --a response explicitly condemned by Christ, and one that has mere blood vengeance as its end. It's mere murder. "Proportionate response" means that the damage inflicted is sufficient to a just end.
In this case the just end is the neutralization of Hezbollah. "Proportionality" thus means that Israel must limit itself to the means sufficient to accomplish that end. Nothing more --but also, please note, nothing less-- is proportional. You don't get to "retaliate" under Just War theory, you get to achieve tactical ends to defend yourself and ultimately restore peace. Peace understood not as cease-fire --anybody can have "peace" at any time by simply surrendering his natural rights to the tyrant-- but a just peace.
And may I add that, knowing that your enemy deliberately uses human shields and plants itself in population centers, if you give up the tactical element of surprise --using leaflets and loudspeakers in order to warn civilians what's coming so they can flee-- you cannot be said to be unjustly targeting civilians. That fault sits squarely on enemy shoulders.
I note that the people who are loudest in decrying the "cycle of violence" generally also encourage an understanding of "proportionate response" that perpetuates precisely that.
UPDATE: John Krenson applies Just War theory to this case. Curtsy to Hugh Hewitt.