Whedon told Wired at SXSW that progress is, in fact, being made on the sequel:
“We are hard on it. It’s been a really tough year — couple of years — partially, I’m happy to say, because of work."
Though he didn't mention much about the plot, he did suggest the most of the original cast would be returning and that it's mostly a matter of scheduling in order to film the next one:
“We’re stuck, basically, at this one part in the process. All of us are looking at this summer as the time when we can go, ‘Hey, you know what? We’ve got a little free time, let’s get unstuck.’ Neil has been like, ‘How long?’ We’re all sort of just waiting, but the waiting will stop.”
I, for one, can't wait.