From here, an interview with Joss Whedon at Comic-Con:
So give me hope for a Serenity II
JW: Hope for it probably rests with this DVD.
Well I’ll buy one.
JW: Actually I will too, people ask me why I would buy my own DVD and I tell them do you have any idea how awesome it is to buy your own movie?
So then you’re saying we’ve got a chance?
JW: Well it’s probably not being discussed in boardrooms right now, but the fact of the matter is if it makes enough money sooner or later they say “hey, this is money!” Also there are paradigms that are much cheaper, it doesn’t have to be enormous. But on the other hand I’m happy to say all of my actors are working very hard. It’s not the same situation where we all threw in for pennies because we had to finish telling that story. Now that situation might be harder to bring about.
That is all.