I AM... not surprised... According to ShockTillYouDrop, Author Richard Matheson has signed off on all sequel rights to his story for I AM LEGEND, meaning that Warner Bros. has every right to look at the Box Office charts, see that it has already made $335 million worldwide in under a month, and decide that they'd be stupider than their own film's ending if they didn't make a follow-up to the damn thing.
This is all the info STYD has provided - obviously no story details are available at the moment. I think this would be a great time, however, for CGI work to commence, since there's no excuse for some of I AM LEGEND'S effects. Just assume there's going to be hundreds of pouncing weirdos climbing up a building or some such nonsense and get to work on it ASAP, lest these things continue to look like characters from MADDEN FOOTBALL.