Hi Marcm,
Thanks for your questions.
You'll find the answers in the How To Guide on obtaining licensing:
viewtopic.php?f=56&t=1253This thread includes the announcement from the Dr Horrible team regarding licensing limitations, so that you can read the details first hand.
It also contains information about limitations surrounding other Whedonverse properties, such as Buffy, Angel, Firefly and Dollhouse. These shows cannot be performed publically (either screened or live performances) as 20th Century Fox do not offer licensing except to broadcasters at this time.
ETA: I've moved this thread to the Advice section rather than the Global Announcements forum.