I’m a Joss fan, and a Dr. Horrible fan, but I’ve heard nothing about any sequels. You need to remember that it was produced during the Writer’s Strike, when both writers and actors basically had nothing to do and were bored of picketing. Now, Felicia Day has The Guild, NPH has How I Met Your Mother, and Nathan Fillion has Castle. And I’m sure Joss is working on something that he’ll eventually sell to Fox, thinking this time will be different. :-P