I found this on the official L4d2 blog, Some achievements contain spoilers about the upcoming campaign so read at your own risk.
  Torch Bearer
Survive The Passing Campaign.
 
 Port of Scavenge
Play 5 full games of Scavenge on The Port.
 
  Fore!
Knock off the heads of 18 infected with the golf club.
 
 Til It Goes Click
Using the M60, kill 25 infected without letting go of the trigger.
 
  Killing Them Swiftly To This Song
Play the new Midnight Riders song on a jukebox.
 
 Wedding Crasher
As the Charger, grab a Survivor and crash them through 8 chairs at the wedding.
 
 Now onto the spoilery ones 
 
  Cache Grab
Open 5 foot lockers.
 
  Kite Like a Man
Kill a tank only with damage from the original Survivors.
 
 Grave Robber
Collect 10 items dropped by a Fallen Survivor.
 
Mutant Overlord
Play 6 Mutations.
 
Link to the blog post:
 
http://www.l4d.com/blog/post.php?id=3721    

So a new mode with collecting Caches and a new mode with Mutants? Or maybe the mutant is a new class and there's just a stage with footlockers? 
 
Edit: Yep, Cache Grab is one of the new modes. Neat. 
Edit 2: No wait, it's too vague to say that. Could mean anything still.

I'm lookin' forward to more zombie slayin'!