It is true that the woman looks and sounds like Summer Glau, and the plot/action of the short clip has some thematic relationship with the TV series Firefly (hence also with Serenity), however, I'm quite certain that this clip itself is not from the movie. I was one of the lucky few who got to see what was billed as a final cut of Serenity at Comic-Con a few weeks ago, and nothing resembling that scene was in it.

I suppose it COULD be some kind of marketing clip from Universal, and does seem to be generating some buzz, but it seems like an odd choice to me.