At the Joss Whedon Dr. Horrible Comic Con panel, Jed Whedon confirmed that there are vague plans in place for a Dr. Horrible sequel. When asked about the posssibility of a next part to the series, Whedon said:

The idea is that there will be another part, so we’re not going to tell you about it yet.

Whedon also went on to discuss the Dr. Horrible DVD:

There is some excitement about a DVD possibility…You’ve probably heard about “Commentary: The Musical”…there are songs written for it. We are also going to have a little contest. We are going to take video submissions for the Evil League of Evil. No longer than 3 minutes, like you’re applying to be on Survivor of something. We’ll put the 10 best ones on the DVD. That’s something we’re going to do after the soundtrack etc.

Some more details: according to Joss, the soundtrack to Dr. Horrible should be available for download within a couple of weeks. In addition, Nathan Fillion revealed that a special website was designed so that Dr. Horrible could use his iPhone as a remote control during the series. In order to convert your iphone into that same remote control, all you have to do is go to: http://thesoftwire.com/horrible_remote.html