Conditional rule?

I've been looking through the material I have on postgreSQL, but can't seem
to find an answer to my problem. Very simplied, my tables are something like
this:

create table news (  id serial,  story text,  publishtime timestamp
)

create table news_unpublished ( news_id
)

I wish to make rule looking something like this:
create rule newsrule as on insert to news do   if new.publishtime is not null insert into news_unpublished
values(new.id);

I.e. "On an insert to news, if new.publish is not null, insert the new
post's id into news_unpublished.

Is this possible?

Thanks

André Næss