Rock, Paper, Shotgun has some
screenshots that are worth a peek. I'll only post one here, so as to steal as little bandwidth as possible:

In addition to
the Charger, a pair of new special infected have recently crept out of the stank. The first is
the Spitter, which has a ranged attack that causes damage within a radius for a period of time. That, I surmise, is Valve's way of addressing the
"shiva-stacking" phenomenon that so plagues the infected team in Versus matches. The Charger is also designed for that purpose, but
accomplishes it by different means. Thus far, there is no special infected known as the Swallower.
The other new special is
the Mud Man, which (I think) shows up only in Swamp Fever. He scuttles around on all fours, which makes him difficult to see amongst a mob, and probably impossible to see in deeper water. As you can see in
this video, Muddy's melee attack covers your HUD with muck, which obscures your vision a bit (though not as much as when a Boomer honks on you).
UPDATE: Shack News has a bevy of
gameplay videos.
LATE UPDATE: The Mud Man is not a special infected, but rather one of the "uncommon common" infected (like the haz-mat zombies) that appear only in certain campaigns.