/Film is reporting that, in response to a question about a sequel for Dr. Horrible, Joss Whedon has said:
"The idea is that there will be another part, so we’re not going to tell you about it yet."Pretty mysterious, but to quote Moist from the movie "It's not a no..."
I for one wouldn't expect a sequel soon, especially because we don't know how long it really took to make Dr. Horrible. Plus, it's always good business to let the epicness of a big success sink in before making a sequel.
Also, if you're interested, there is a planned soundtrack and the DVD is on track to come out too.
Edit: The soundtrack is available online from both iTunes and Amazon. If you plan to get it from Amazon, use the link at the left and support Mason Tech Beat!