I want to see I Am Legend and my 8 yr old nephew hasnt said anything about it. He loves horror movies. Freddie Krueger, Halloween, he's all for it. He totally understands that it's all fake. When he was five or so and saw LOTR for the first time he turned to his mom and said, "Don't be afraid, it's just a guy in a suit." Kissing, however, horrifies him and he won't watch it. And yet, he was the only boy to dance with a girl in the christmas concert. It depends on the kid what you'll let him see. I think my 15 yr old nephew is still scared when he sees LOTR, the 8 yr old thinks it's all old hat.
I think if you're going to let a kid see a scary movie for the first time, it should be a situation teh child wouldn't find himself in so he's not going to be worried in his real life that somethign scary will happen. So no monsters from teh closet or under the bed. But Jaws? Or Aliens in space? Totally safe.