Mozilla Messaging has completed the merge of Thunderbird 12 to the Early Bird developmental channel. Existing Earlybird user may be able to update within the application via Help > About Earlybird (at press time the updater was not working) otherwise you can manually download and install Earlybird from here. Thunderbird 12 is scheduled for release on April 23, 2012.
Lightning Users: You will need to manually update to Lightning 1.4b1 via the Calendar Versions site.