This is very annoying as Evolution events were not only visible, but also clickable in the unity panel calendar. Isn't this possible with Thunderbird in Oneiric?
Thunderbird – Why Lightning Events Not Showing in Unity Panel Calendar
indicatorlightningthunderbird
Best Answer
The calendar in the Unity panel is hard-coded to work with Evolution; however, progress is being made on fixing this. See this post on Mike Conley's Blog.
In short, Lightening events in Thunderbird can be shown in the 'indicator-datetime' panel calendar, but the plugin is not finished or released yet. (For example, clicking an event doesn't automatically open the event in Thunderbird)