Gabe Newell had this to say:

Condition Zero is going gold this Friday, and will be available via Steam and at retail on November 18th.

The AI that we were testing via the Steam beta has been more successful than we had originally expected, so it has allowed us to build some more flexibility into the mission authoring.

A typical mission in CZ looks like this:
- you have to win at least three times, and have won at least two more than your opponents
- in easy mode, you have to get one kill with a pistol
- in normal mode, you have to get one kill with a five seven
- in hard mode, you have to get three kills with a five seven without dying which may take multiple rounds
- in expert mode, you have to get three head shots with a deagle without dying.

Mission MODers (I can't think of a better name off the top of my head) will be able to create their own missions. The UI and gameplay is data-driven, so once the missions are installed, they automatically appear.