Before you ask, "modes" is indeed supposed to be plural. The Passing was originally due to launch in late March, but it seems that may not happen with only a week left in the month. Nonetheless, it should be here before the summer.
Along with sharing additional details about the upcoming DLC, Valve has also reduced the price on Left 4 Dead 2. Between today Thursday, you can grab the title for 50% off, or $24.99. That discount goes for the four pack too, which works out to only $18.75 per copy. If you don't have four friends interested in splitting the bundle, feel free to express your interest in the comments and perhaps someone will bite.