For those of you using Thunderbird 12 betas with Lightning and having trouble with 1.4b2 (calendar views empty, lots of errors in the error console), please do upgrade to Lightning 1.4b3. This error shows up with certain locales, that were wrongfully included in 1.4b2 but not ready for production yet. The situation is described in more detail in bug 736717.
For those of you using the Provider for Google Calendar, this beta release (yes, in Lightning, not in the provider) also fixes an issue that causes events in secondary calendars to be moved to the primary calendar on edit.
The final Lightning 1.4 as well as a new Provider for Google Calendar release is due on May 1st, 2012 and will include mentioned fixes, in addition to numerous other bugs.