Problems with GET DIAGNOSTICS SELECT PROCESSED INTO result;

It doesn't seem to work as advertised:

DROP TABLE junk;
CREATE TABLE junk (something INT4);

DROP FUNCTION testfunction(INT4);
CREATE FUNCTION testfunction(INT4) RETURNS INT4 AS '
DECLARE
    result INT4;
BEGIN
    INSERT INTO junk (something) VALUES ($1);
    GET DIAGNOSTICS SELECT PROCESSED INTO result;
    RETURN result;
END;
' LANGUAGE 'plpgsql';

When I then run it I get:

=> select testfunction(10);
NOTICE:  plpgsql: ERROR during compile of testfunction near line 5
ERROR:  parse error at or near "SELECT"

Any idea how to get this to work?  This is with PG7.1.2.

Thanks