Long term database archival

Hi,

What is the best pg_dump format for long-term database
archival?  That is, what format is most likely to
be able to be restored into a future PostgreSQL
cluster.

Mostly, we're interested in dumps done with
--data-only, and have preferred the
default (-F c) format.  But this form is somewhat more
opaque than a plain text SQL dump, which is bound
to be supported forever "out of the box".
Should we want to restore a 20 year old backup
nobody's going to want to be messing around with
decoding a "custom" format dump if it does not
just load all by itself.

Is the answer different if we're dumping the
schema as well as the data?

Thanks.

Karl <kop@meme.com>
Free Software:  "You don't pay back, you pay forward."
                  -- Robert A. Heinlein