Seems like a bug to me.
PostgreSQL 7.4 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.2 20030222 (Red Hat Linux 3.2.2-5):
my_example=# select has_function_privilege('postgres', 'cash_cmp', 'execute');
ERROR: expected a left parenthesis
my_example=# select has_function_privilege('cash_cmp', 'execute');
ERROR: expected a left parenthesis
Stefan