Windows/Mac/Linux: One of the notable absences from the launch of Thunderbird 3 was a compatible version of Lightning, an integrated calendar tool. Google and other calendars return to the 'bird with beta builds of Lightning and Provider for Google Calendar.
Lightning, the calendar add-on, and Provider, a plug-in that provides two-way syncing with Google Calendar, weren't compatible with Thunderbird 3 when it launched. These betas are.
Need help getting your Google account synced up to Lightning? Adam covered it in his guide to syncing any desktop calendar with Google Calendar.
Lightning and Provider for Google are free downloads for Windows, Mac and Linux systems. I had some compatibility trouble trying to load the extensions in a daily build of Thunderbird in Ubuntu, but others are reporting successful installations in Linux.