Yes, a Firefly MMOG (Massive Multiplayer Online Game) is being developed. It is scheduled for release sometime in 2008 but knowing MMOGs it could be later or never. If comes to life I am sure it will be flooded with thousands of Whedon / Firefly fans the instant the doors open. If the game then lives up to the quality of the show or movie it will certainly be a hit.
Due to the rabid cult following of the TV show Firefly (and movie Serenity) interest in Joss Whedon and the Firefly story live on. Take a look at WHEDONesque for more info.