You're thinking of the article on The Director from the L4D2 wiki, which claims:
In Left 4 Dead 2, the Director is
sensitive to the amount of teamwork
going on as measured by how many times
teammates heal each other and
rewarding more teamwork with more
health kits throughout the map.
The director first populates the map with pain pills in predetermined locations according to the pain_pill_density cvar, which means that when the survivors are in the safe room initially, there are no health kits anywhere in the outside world. The director has the power to change these pain pills into health kits based on the performance of the team, modified by the director_convert_pills variable. This is why on harder modes, you'll see less health kits, even if the team is cooperating well.
So the answer to your question is yes, there is a "karma" value, though it's not known by that name.
Don't take my word for it, though. Play through Dark Carnival a few times and check the first aid station in the small room just before the coaster. You'll get different amounts of medkits and pills each game, based on your performance.
For more detailed information, see this presentation by Valve's Mike Booth: