Audit-trail engine: getting the application's layer user_id

Hey guys,

I needed to implement an audit trail engine and decided to do it on the
database layer.

I already have a basic but fully functional audit trail system implemented
on my PostgreSQL 8.2 server. It has been done using PL/PGSQL
and triggers and it works pretty well.

Here's what I need to do: Somehow save the user_id of the **application**
user who have done the update/delete action to the log row.

Mine is an web application - three tier. The app connects to the db using
only one user and it has it's own authentication system and doesn't
rely on the database for user management.

I've read an article on IBM's developer site which teaches how to do just
that (get the application's user id and save it the audit
row) using what they call the APPLICATION_ID which is an unique ID that DB2
assigns to the app when it connects to the database. A
relation table is then created to relate the user_id and application_id.
When the user logs in, a new record is created in this table, and,
as the application_id is available in the db enviroment to the procedural
languages, we can then go to this table and finally get the
user_id. Is there something like that in Postgres?

Thanks in advance!

Marcelo.