Loved Firefly and thought Serenity was a great though under appreciated movie.
I would think that straight to DVD would be okay since the first one didn't do too well in box office but it did so well when it was released on DVD that I think they have sufficiently increased their fans and attracted enough attention to warrant another attempt at the box office since most movie goers are blockheads who prefer flashy uninteresting movies and underachieving sequels anyway.
Either way its great news hope it comes to fruition.