Um ... I disagree with O'Neill on _one_ point. Planets with a working
ecosystem can be engineered to be reasonably fail-safe, to the extent
of surviving a collapse (or even extinction) of civilization. A habitat
could _conceivably_ be built to such a scale (go re-read Ringworld, for
example), but most designs for space habitats are way too small to be
stable -- much less to withstand the slings and arrows of existence in
a planetary system for long periods of time (megayears).
If you hit an O'Neill cylinder with a rock, it dies. If you hit the
Earth with a rock -- anything much smaller than Phobos -- well, a few
species may go extinct, but the whole system will pick itself up and
carry on as before.
Of course, why the hell are we talking about habitats to support an
agriculture-based society when there's the open question of what kind
of habitat is appropriate to an uploaded mind to discuss?)
-- Charlie