Ian,
> Debian Woody with 2.4.18 Linux kernel. Postgres install from apt-get
> (7.2.1).
Hmmm. I might suggest polling both the Debian mailing lists and the
pgsql-hackers mailing list. I remember vaguely hearing about some bug with
Postgres on Debian, but I can't remember where I heard it.
It's also possible that one or more of the accessory applications you are
using is playing "fast and loose" with the filesystem, and in the process
damaging some of the Postgres files.
--
-Josh Berkus