Possibility
First off yes the book and movie are not a like at all, and frankly I don't care. It sucks that they use the name of this novel and the character names, while it is nothing a like, and yes it sucks that they haven't made a film that stays true to the novel, but the film is very good in my opinion (spectacular compared to everything else that is coming out anymore), and they are so different it is basically like 2 completely different stories, it would have bothered me more if they had tried to make them similar and then totally screwed it up. Anyway, a sequel could be possible if writers do things right. First they can't try to put Robert Neville into it somehow surviving or anything like that, even if they do some terrible flashback where it shows the alternate ending released on the DVD as being what really happened, and the original ending just being either a nightmare, or fleeting thought Neville had before he blew himself up. The trick would be just to forget Neville in the second movie, make the protagonist completely different from Neville in terms of motivation. For instance Neville was obsessed with a "cure", our new protagonist could be bent on revenge or extermination. The story won't be as good, but the action and intensity could be extremely enthralling. They could also introduce a whole second type of infected that are more like the dominant male from the original movie. More emotional, slightly less fearful of light, and more intelligent. Of course set it somewhere else. The ending could revolve around a group of survivors coming to the area to cure the infected and thus totally altering the entire scenario this protagonist has based his actions on. Also seeing those that he had tried to exterminate (and maybe even tortured), becoming cured sane individuals could drive him to insanity. I don't know its just an idea, but a better possibility than "oh wait Neville survived!" or replaying the whole plot just in a different city with a slightly different protagonist. One could also argue they could make a film from the perspective of the infected, it would be extremely disturbing, but may make for a good movie if done right.