Conditional foreign key?

We have a list of customers, some of whom have purchased feature X and some of
whom have not. If a customer has paid for featurex, they can use it, and a
strict relationship between cust_items.items_id and items.id, but only if
they are signed up to use featurex, otherwise I want cust_items.items_id to
be NULL.

Currently, I have tables defined similar to:

create table Customer (
id serial unique not null,
name varchar(30) unique not null,
FeatureX bool not null
);

Create table cust_items (
id serial unique not null,
customer_id integer not null references customer(id),
name varchar(30) not null,
type varchar not null,
items_id integer default null references featurex(id),
cust_active bool not null
);

// type is one of "book", "tape", or "featurex"

Create table items (
id serial not null unique,
title varchar(30)
);

I want to say
"If the cust_items.type='featurex' then (
(customer.featurex must be true)
AND
(cust_items.items_id must be in
(select id from items)
)";

I'm just stumped as to how to say this.

I've tried, with the above table defs,
CREATE RULE check_customer ON UPDATE to cust_items
WHERE NEW.type='featurex' AND
NEW.customer_id IN
(SELECT customer.id FROM customer
WHERE featurex=TRUE
)
DO ... ? <too many tries to count>

Any pointers, hints, or info on this kind of statement?

Thanks,

Ben

--
"I kept looking around for somebody to solve the problem.
Then I realized I am somebody"
-Anonymous