Help! Can't pg_dump anything: handler procedure for procedural language plpgsql not found

Hi there,

I wonder if anyone can shed some light on a very frustrating problem.

I'm running a debian linux 3.0 "woody" server, nothing special, with
the latest version of postres that apt-get will allow me, which I
*think* it;s 7.1 something, I don't know how to figure out the
postgres version. Anywho - I'm trying to backup my databases, which I
did at one point, but I have no idea what happened, could have been an
upgrade. My Dbs have large objects in them, so I would do the
following as per the man pages:

  pg_dump -S postgres -Ft -b mydb > mydb.tar

But i'd always get the following error message:

 pg_dump: handler procedure for procedural language plpgsql not found

And even with the most simple of pg_dumps, I would still get that
error, with a 0 sized file to show for it.

Does anyone have any ideas on how to fix this?

Thanks,
  Mark