Since we're past feature freeze does that mean that there will be no more efforts into integration of Thunderbird and Unity?
According to this article, Thunderbird has already achieved complete integration with all of the special menu schemes in Unity. Even if 11.10 ships without complete integration, it shouldn't be any trouble to add it in yourself. You'll either be able to manually modify it, or add in a Mozilla PPA which includes a post-feature-freeze version of Thunderbird and Lightning.
It's already quite possible to add Thunderbird to the time/date indicator in Ubuntu: these instructions will take you through it step by step. They also explains how to thoroughly integrate Thunderbird with your Ubuntu desktop.
Since we're in feature freeze, there aren't likely to be major changes to the calendar indicator design before 11.10. This should make it easier for the Thunderbird developers (and independent add-on developers) to update their Unity-ready addons for 11.10. If you're looking for other unity-specific ways to make Thunderbird play nice with Ubuntu check out these three addons:
Note that they may become unnecessary, if Mozilla continues to add Ubuntu/Unity functionality directly into Thunderbird. This AskUbuntu answer gives some other ways that you can customize your Unity Thunderbird launcher.
If you're feeling particularly adventurous, and want to try out the latest and greatest versions of Thunderbird right now, you can try out one of the Mozilla personal package archives: