I can tell you that this feature will be part of our v3 API; however that has not yet been released. We will be announcing it soon, but I do not have a date right now.
We are doing what we can to make the transition as easy as possible. There will of course be some things that are not easy to move over, but we will try to make it smooth if we can.
Was this ever implemented? Seems very limiting if not since all the other major email providers offer this. I'm trying to implement integration between Gravity Forms and CC and would like to tag the audience vs create a new list.
We do still have plans to bring support for Tags to our V3 API, though it is not yet released. In the meantime, we recommend using custom fields which can be used in the website UI to create custom segments separate from lists. See this link for more details about segments: http://knowledgebase.constantcontact.com/articles/KnowledgeBase/25205-Define-Your-Own-List-Segments
Please let us know if you have any other questions!
Tier II API Support Engineer