Whedonopolis has a report from a Zach Whedon signing where Jed Whedon stopped by and announced:

Drum roll: YES, Dr Horrible is in the process of being made.  It is now being organized and written [WRITTEN!!!]. In fact, while I was talking with Zack, Salena spoke with Jed who confirmed that he’s there to meet with Zack and start writing the script this afternoon.