I should amend the above: Lightning also sends meeting information (invitations/revisions/replies) only as email attachments, rather than through CalDAV. I recently compiled information on this for another forum, so I might as well share it here.
See
Nabble - Mozilla - Calendar - Caldav
Also
Calendar Weblog: Bruno's developer notes: CalDAV Scheduling Needs You!
And
Calendar Weblog: Lightning 0.9 and Sunbird 0.9 are out! The Rumbling Edge :: Sunbird 0.9 Released (list of bugfixes and enhancements in 0.9)
https://bugzilla.mozilla.org/show_bug.cgi?id=409921 (discusson of CalDAV sched support--nixed for 0.9)
https://bugzilla.mozilla.org/show_bug.cgi?id=452610 (decision to disable CalDAV sched for 0.9)
https://bugzilla.mozilla.org/show_bug.cgi?id=454998 (notwithstanding lack of CalDAV sched, Free/Busy *is* supported in 0.9)
For reference, when I write "caldav-sched", I mean
http://www.ietf.org/internet-drafts/...v-sched-06.txt
Technically, Free-Busy is part of caldav-sched, but the Mozilla folks refer to them separately, basically because they've got FB working but not the other parts. They apparently hope to get the rest working in Thunderbird 3.0 (which will integrate Lightning by default).