In order to pull off the scene properly, the second-time director needed the assistance of one bridge (Brooklyn!), 6 nights, 14 government agencies, 160 National Guardsmen, 250 crew people, one thousand extras and probably 10,000 cups of coffee. Oh, and about five million bucks, although apparently some internet folks* believe the sequence cost a whole lot more than that. No word yet on how lengthy this sequence might be, but for that kind of money (and effort) it better be pretty impressive. At the very least, I'm sure the six nights spent on the Brooklyn Bridge will make for one really nifty featurette on the I Am Legend DVD.
(* Both Yahoo and The Hollywood Reporter cite anonymous "internet" reports while claiming that the sequence may have cost a lot more than $5 million, but I was unable to track down any such reports. Cite your sources, journalists!)