[This was posted to a mailing list I'm on; as soon as I get a link to reference the original quote, I'll edit this.]
To quote from the Whedonesque site:
"Here's a thing: when "Firefly" was cancelled, my heart got broke.
Sounds a bit much, but it changed me. Not even "Serenity" could
patch that wound. I'm wearier, warier -- after all those years as a
movie writer, you'd think I'd be prepared for another lesson on my
unimportance in the scheme of things, but I wasn't. There are two
very separate worlds: the marketplace, and the bustling bazaar that
is my brain. The brain place is crowded with goods, ideas, sequels,
spinoffs, animated versions, miniseries, radio dramas -- this is
just the used goods. All the new wares are in there as well and it's
deafening. Once I create a verse I never let go of it. And figuring
out how much of my energy should be devoted to reawakening the
projects you all love with the actors and characters I all love, and
how much should be forging ahead and creating entirely new works
(which you are contractually obligated to love) is exhausting. More
than you know. You know the horse caught bwtween two pools of water?
Add seven pools, and make the horse wicked A.D.D. The other world,
the marketplace, I don't even begin to understand or predict. All
these rumor of projects or the death of projects... When the two
worlds align and something actually happens, whatever it is, you
guys know I'll be on this site as soon as I'm allowed to be. And
I'll be very very clear. There is no news. Not never, just now. I'm
off to lunch with Lonelygirl."
Your Scribe, -j."
So, basically, the "Serenity is dead!" stuff was taking him out of context. No, there's nothing planned now, but that doesn't mean Joss doesn't want to work on something else Serenity related in the future. Personally, my first choice of media format is tv, maybe on Sci-fi. Next choice is actually novels; after reading the novelization of the movie, I think a novel actually fits the style the right way. I want that episodic feel; this isn't a concept suited to a movie, I think.