Valve writer Chet Faliszek has revealed that the studio was supposed to announce upcoming Left 4 Dead DLC before Christmas, but the press release was postponed for whatever reason.  Luckily, the delay only applies to the announcement itself, which should be made "shortly," and the content should be right on schedule -- whenever that is.

Faliszek also confirmed that the Left 4 Dead DLC will be released simultaneously for the PC and Xbox 360 versions.  Hallelujah!