A while back, I posted some comments by Firefly/Serenity creator Joss Whedom regarding the chances of another Serenity film.

Now one of the actors is talking about it:
Alan Tudyk, who played Wash, made the following comments to Moviehole.net:

They had to put [the new DVD] out because they’ve been selling out of the other one and so Universal’s like ‘So, let’s do another one’. And now… there’s now a chance there’s going to be another movie”.
Tudyk agrees that even if it was a direct-to-DVD movie, it’d still be worthwhile. Especially since the whole DVD sequel is a big trend.
“It really is”, says Tudyk. “Everybody in the Firefly crew – and that includes the ones who died in the movie – are excited about the prospect of doing another”.

So clearly it is not a done deal by any means, but there is some talk within the studio and cast. Any hope is better than no hope.