Left4Dead SDK coming "shortly"

wilsonwiz asks: Have you any idea when the Source Development Kit will be released?

Chet Faliszek: Shortly. I know we are working on it and want it out as soon as possible. I really think this is a game that brings back that urge to make maps like Doom did back in its day. You want to make your place of work and unleash the zombie apocalypse on it. When we release the SDK we will also be releasing tutorials for level makers, from newbies who have never used hammer to experienced mappers.

Left 4 Dead live interview