It is PLANNED... no release date specified yet and a plan doesn't mean it will ever be released.
Considering its been designed for great customisation and modding capability (aka lua and xml format for most files) and stated by the Community manager that it'll be out sometime next week, it would be kind of stupid for Civ V not to have it.
I agree it would be a bad move to pull the plug on it but it's hardly the first time modders were promised big things during game development and after release just to get nothing later.
This time I doubt Firaxis will fail to deliver the goods. One of the dev team was taken in from the modder community and that gives us high hope that this SDK will not only be a reality but it will be great.