Valve announced "The Sacrifice," which is the prequel to "The Passing," explaining how Zoey & Co. got down to
There is good news to be had with this announcement. The Sacrifice will work for both L4D and L4D2, so even if you don't have numero uno (and seriously, why not?), you can still experience what had happened.
Finally, Valve will be releasing, alongside The Sacrifice, a L4D2-compatible version of No Mercy, L4D's first chapter campaign (and easily the better of the bunch).
It seems that the entire story may not be fully revealed in The Sacrifice, as plans for a digital comic to be distributed next month is set to get players up to speed on the story of the new DLC.
If you have your zombie-shooter on a computer-type device, then you get The Sacrifice for free. If you have the 360, expect to pony up for it (no price was announced, but I'm expecting it to be $7 like it has always been).