Q: insert/update in same statement

Is it possible to make a single SQL statement that will insert a row
if it does not already exist, otherwise it will just update the
fields?

Actually, just a statement that will not give an error about duplicate
keys would be enough for my application.

Something like

INSERT INTO SOME_TABLE (FIELD1, ...) VALUES (VALUE1, ...) WHERE
KEY_FIELD DOES NOT EXIST;

I am working in PostgreSQL 7.3.x.

Thanks, Ralph