auto-increment integer field for each row

Greetings,
I'm trying to create a trigger which will auto-increment the integer
stored in one of the 'not null' table rows each time that row is
updated.

I tried to use the trigger example here, but it silently fails to work:
http://archives.postgresql.org/pgsql-sql/2000-02/msg00147.php

I can repeatedly update column 'b' in the table of that example, and
'a' will remain '1'.

I'm guessing that maybe plpgsql has changed sufficiently since that
example was written (almost 10! years ago), that its no longer
compatible with the version (8.1.10) that I'm using.

many thanks for any advice/pointers.

-Lonni