I´m running gentoo linux x86_64 on my dell laptop. Today mail-client/mozilla-thunderbird-3.0.3 was aviable in portage and so I upgraded, but there was no lighning for Thunderbird 3 in the portage. Since I need the calendar in thunderbird I searched the web for a solution. After short time I found it in google groups.
On Gentoo just do this:
emerge =mail-client/mozilla-thunderbird-3.0.3 =x11-plugins/enigmail-1.0.1-r1
Download the lightning.xpi from http://mozilla.osuosl.org/pub/mozilla.org/calendar/lightning/releases/1.0b1/contrib/linux-x86_64/ and install it via Tools->Addons.
Now have fun with your new Thundebird.
Best regards xexplorer