I think people are confusing the monster-type in "I Am Legend." I don't think they were actually zombies. From my limited experience with zombies (Resident Evil, Diablos I and II, Shaun of the Dead), they are reanimated corpses (otherwise known as "undead"), extremely unintelligent, and extremely slow (their power is in their numbers in spite of their slowness). With the monsters in "I Am Legend," they never died - it was more a parasitic virus that overtook all brain activity and all activity on the cellular reproductive level. Another two reasons why I don't think that these creatures were "undead" is that Sam didn't die before she became one of them - it was more of a slow progression to becoming that monster. That, and at the end of the movie, Smith's character actually cured the captured monster of the affliction. There's These creatures also were extremely intelligent - IIRC they set up traps and cornered Smith's character on several different occasions. Also, these guys were extremely quick - faster than the humans even.
Actually, I think they resembled more rabid-like humans than zombies. But I get the point... :wink: