Jeff and Josh,
I found this example in "Practical PostgreSQL"... will it do the job?
"""
The following example defines an aggregate function named sum(), for
use with the text data type. This aggregate calls the
textcat(text,text) function built into PostgreSQL to return a
concatenated "sum" of all the text found in its input values:
booktown=# CREATE AGGREGATE sum ( BASETYPE = text,
booktown(# SFUNC = textcat,
booktown(# STYPE = text,
booktown(# INITCOND = '' );
CREATE
booktown=# SELECT sum(title || ' ') FROM books WHERE title ~ '^L'; sum
-------------------------------Little Women Learning Python
(1 row)
"""
...Edmund.