Apparently, during the writer's strike, Joss started writing a musical...
Here's a quote from the 'official fan site': (dated back on March 16th)
"Joss Whedon announced today that Nathan Fillion will play the hero in a special Internet Musical.
Whedon reports that during the WGA strike he started writing the musical which will be a limited internet series. Each of the three episodes will be approximately ten minute each.
Co-writers for the internet feature are Joss’ brothers Zack and Jed and Jed’s Fiancé Maurissa . The writing has been completed and shooting commenced today.
“It’s the story of a low-rent super-villain, the hero who keeps beating him up, and the cute girl from the laundromat he’s too shy to talk to.” Says Whedon.
“Doctor Horrible’s Sing-Along Blog” will star Neil Patrick Harris as Dr. Horrible, Nathan Fillion as Captain Hammer, Felicia Day as Penny and a cast of dozens."