Joss Whedon confirms that there aren’t any plans for a sequel to Serenity, thus:

There’s no sequel, no secret project regarding Serenity or somesuch and I’m not even sure how anyone thought there was talk there. I’ve seen Nathan and Tim (and Summer and Alan) recently because they’re my friends because I’m so, yeah, awesome. So let’s put that to bed and smother it with a pillow.