http://www.rockpaper...ids/#more-25478
Some details they've given:
- New weapons - a heavy machine gun (but like the Grenade Launcher or Chainsaw, when it's out of ammo, that's it) and the golf club.
- New Midnight Riders song.
- New uncommon common infected: the fallen survivor - runs away but drops healthpack/goodies when killed.
Coming out at the end of March (Valve Time

And... AND:
New L4D1 DLC coming! Looks like you're in luck, Slow Ben
