Pull up a chair, children.
In previous interviews, Joss has said that, should Firefly and/or Serenity return, so will Wash & Book, but "not in a silly, 'long lost twin' kind of way" (his words). Moreover, while a massive selling of the Serenity DVDs might be enough to ressurrect the idea, it is highly doubtful that Firefly will return. 'Why?' You ask? Simple; FOX. They still hold the rights to the series and it is entirely up to them to bring it back. Given the run they gave it when it first aired, most soundly believe that FOX wouldn't even bother bringing it back.
Thus, as stated in a few Whedon interviews, the Firefly crew will likely return in the form of movies, most likely straight-to-DVD, which is fast becoming a lucrative industry (especially for Universal) with rising movie-costs and falling Hollywood quality.
So, while the series returning may be a very shiny prospect, there is a general consensus of level-headed browncoats that it just won't happen... but hey, we can still hope the FOX executives get hit by falling space debris, can't we?
@Ford Perfect; Book died when his small refuge was attacked by the Alliance, more accurately a group of Alliance troops under the command of the Operative. His corpse was then, unceremoniously strapped onto the front of Serenity.
