People, have you seen the stunt rehearsal for the River fight scene in the Maidenhead? It's not Summer Glau in the rehearsal, but you can actually see how much River might have kicked butt if we could have seen more (or if they shot more with Summer). The stunt woman is very quick even if she doesn't have a third of Summer's grace, but damn this was something to watch.
Click on the stunt reels link and it's the last video (Sample Fight Scene). And then imagine that it was River Tam doing all of this. Just, you know, more gracefully. :)
And while you're there, check the other videos, especially the one above the Sample Fight Scene. It has some more Serenity stunts.