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?
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)