Updating & inserting in one shot!

In my Postgresql 8.06 db I have a table in which the key fields are a
date and a code.

Now I need to load data from a text file (or a
temporary table) that either should update OR should be insert-ed into
that table.

Is there any ready-to-use, contributed function allowing
this 1-shot update-insert into a table?

Ciao
Vittorio