empty dates and changing the default date behaviour

Hi,

First of all I want to congratulate all the people working on PostgreSQL for a job well done.

Now my problem.

I want to change the default behaviour of the data_in function without recompiling postgres.
the functionality is the following.

create table test (d date);
insert into test values ('');

this should result in the field d containing NULL and the insert command should work without returning an error.

Is this possible and how do I do this.