Looks like along with Scavenge mode in Left 4 Dead 2, Valve has added a hardcore mode called Realism.
Noticed by 1UP in the menu options during a preview, VP of marketing Doug Lombardi explained to the site that the mode was something for the "super hardcore player".
"Take Portal for example," he said when describing the mode. "We had our best internal players who could blow through that game in about 30 minutes, but within weeks of launching it we had people on YouTube beating [the testers'] times by 10 minutes or more.
"Realism is for those players."
Via Evil Avatar.