trying to learn plpqsql... so please forgive..

Maybe this should be sent to novice... I was not certain, but if it should, 
please tell me so.

The matter at hand is this:

When I do an 'INSERT INTO <table> VALUES <row1,row2,row3>'
and on the table is a serial primary key named p_key.
As I want this number to be auto-generated, but use it as a 'customer 
number', I want to create this function to return the value of this insert. 
I thought/hoped that this would work, but as there are some people 
dependant on this database, I dare not try out too much ;-) This would be 
the first time I used plpgsql, so I am not so certain about what I do.

CREATE FUNCTION add_cust() RETURNS INT4 AS ' -- SERIAL data type is really 
an INT4 (and some more).    BEGIN        RETURN NEW.p_key;    END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER add_cust BEFORE INSERT ON table    FOR EACH ROW EXECUTE PROCEDURE add_cust();


Someone willing to evaluate this for me, and telling me if it is safe to 
use as it is? or things I may do wrong?

TIA,
Michiel