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