That "September 23" date was obviously wrong, but not by much, as Valve announced today that Left 4 Dead's Crash Course DLC will be out next week.
September 29 is the date you'll be able to play through the new campaign on both 360 and PC, which acts as a bridge between the No Mercy and Death Toll campaigns found in the main game.
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The 360 version will cost you 560MSP, while the PC version is free.