Why "ERROR: dtoi4: integer out of range" on pg_dump

Folks,

We have a database with several very large tables.  When trying
to pg_dump we get the above error, e.g.:

pg_dump -v wsdb
--  saving database definition
--  last builtin oid is 18539 
--  reading user-defined types 
--  reading user-defined functions 
--  reading user-defined aggregates 
--  reading user-defined operators 
--  reading user-defined tables 
getTables(): SELECT (for PRIMARY KEY) failed on table v3otgdsrcq.
Explanation from backend: ERROR:  dtoi4: integer out of range

Making another small database (same system, 7.1.2 on Debian/GNU Linux
2.2), gives the same sort of problem:

pg_dump -v tmp
--  saving database definition
--  last builtin oid is 18539 
--  reading user-defined types 
--  reading user-defined functions 
--  reading user-defined aggregates 
--  reading user-defined operators 
--  reading user-defined tables 
--  reading indices information 
--  reading table inheritance information 
--  finding the attribute names and types for each table 
--  finding the attrs and types for table: 'tmp' 
--  flagging inherited attributes in subtables 
--  dumping out database comment 
DumpComment: SELECT failed: 'ERROR:  dtoi4: integer out of range

If I init a new db and restart postgres with the new base,
no problem.

I suspect some sort of corruption but we're not sure where to
look.  A vacuum did not help.  We'd like to recover, if at all
possible.  Any ideas (no luck on other lists or I wouldn't post
here)?

TIA,

--Martin