Glad to see that the v3 API is out!
I can't find details anywhere about new tag features in v3. Is there any way to add tags to new or existing contacts with the new API?
Our v3 API has limited release. The contacts endpoints are available, but as you noticed there is currently not a way to add tags. Our engineering teams are comparing the use of tags versus segmentations in the UI and will implement which ever one does better in the v3 API.
While there are no current plans to retire the Tags feature, we don't have immediate plans for any new development involving it at this time. Our v3 API does have advanced Custom Fields features, which work well with our new Segments feature allowing you to easily segment out your list and replicate the behavior of Tags.
See here for details on our Segments feature: http://knowledgebase.constantcontact.com/articles/KnowledgeBase/25205-Define-Your-Own-List-Segments
Tier II API Support Engineer