Can I use check-constraint to set null?

(Postgres 8.4.2)

CREATE TABLE test (foo numeric(8,2) CHECK (foo >= 0.0));

ecdb=> insert into test values (-2.2);
ERROR: new row for relation "test" violates check constraint "test_foo_check"

This is expected.

However, is there a way to insert a null when this check fails,
instead of returning an error (and thus aborting the insert)? I'm
reading on http://www.postgresql.org/docs/current/static/ddl-constraints.html
but cannot find an answer to this question.

--
- Rikard