Horrible Wiki

At Comic-Con '08 Jed Whedon confirmed that a sequel is being planned but does not have a confirmed release date as of yet. Later, it was revealed it was planned to be filmed during the summer of 2012

In the June 2010 edition of Nintendo Power, Felicia Day was interviewed, and the sequel mentioned.

Interviewer: Speaking of which- has there been any movement on a Dr. Horrible sequel?
Day: I heard they're actually starting writing now, which is awesome! Cross fingers Penny can be included.
Jed Whedon talks to the LA Times about Dr Horrible’s Sing-Along Blog

Q. There’s been talk of a sequel. How far along is the planning for it, if at all?

JW: We have some songs, a rough outline, and no time. Though I have a feeling that when Joss is finished making his massive, expensive superhero movie, he will want nothing more than to make a tiny, cheap superhero movie. With songs.

Q: We’ve seen some of the other villains in the “Dr. Horrible” universe — well, a quick scene — would it be possible to see other heroes in a sequel?

JW: At the risk of spoiling — yes. There will be other heroes in the sequel. Perhaps even a council of them. A Council of Champions, if you will.

on July 2011, Joss Whedon was on Comic Con.

Q: Can we expect to see a Doctor Horrible 2 anytime soon?

JW: The thing that you have to understand... is yes.

There has also been word that they are trying to get Dr. Horrible 2 to debut on the 5th Anniversary of the first movie (June of 2013)

Obviously, that did not happen.