running aggregates entirely in SQL?

I could have sworn that I saw an email from Josh Berkus on sfpug about how to
do running aggregates (ie. a running tally or total, say in an invoice)
entirely in PostgreSQL, however googling only finds:

http://archives.postgresql.org/pgsql-novice/2003-09/msg00305.php

and I can't find it in the sfpug archives:

http://archives.postgresql.org/sfpug/

am I mistaken?

If so, is there a way to keep a running tally entirely in PostgreSQL?

Thanks,
Adi