Amid rumblings of mid-August inventory-clearing, no fewer than five new titles hit the multiplexes this weekend. But for most of us there were only two questions that Friday's box-office results needed to answer: how high could Neill Blomkamp's "district 9 climb and how far would Stephen Sommers' "G.I. Joe" fall? As for the former, the Peter Jackson-produced sci-fi flick parlayed its great word