SETOF and language 'plpgsql'

How do you return a "setof <datatype>" from a plpgsql function?  I
thought the below would do the trick, but it does not.

   create table tbl_foo (id bigint);
   insert into tbl_foo (id) values (1);
   insert into tbl_foo (id) values (2);

   create function func_foo () returns setof bigint as
   'BEGIN
    return (select id from tbl_foo);
    END;
   ' language 'plpgsql';

   select func_foo();
   ERROR:  More than one tuple returned by a subselect used as an
expression.

but this works as expected with "language 'sql'":

   create function func_foo () returns setof bigint as
   'select id from tbl_foo'
   language 'sql';

   select func_foo();
    ?column?
   ----------
           1
           2
   (2 rows)


Either I'm missing something really silly, or plpgsql just can't do it.
Any insight will be greatly appreciated!

eric