I just moved from Thunderbird 2 to 3, using the excellent mozbackup tool to backup/restore (I was also changing machine as well).
Lightning only works in beta with Thunderbird 3, and Google Provider fails. I followed various guides, but none worked, until I used the en_US version of things from
http://releases.mozilla.org/pub/mozilla.org/calendar/lightning/releases/1.0b1rc1/win32/en-US/
Just grab the two .xpi files and add them to your Thunderbird 3 addons.
