If you keep this up, we're going to send you a bill <s>.

This is a situation for a CROSS JOIN.

(Warning - this is straight off the top of my head)

SELECT x.a, x.b
FROM
(SELECT a.value, b.value FROM a CROSS JOIN b) AS x
LEFT OUTER JOIN j ON x.a = j.a_value AND x.b = j.b_value
WHERE
j.id IS NULL

You can play with removing the derived table.

-Mike