Actually, this is NOT 100% true, depending on what CCNP exam track you are on (640-6xx exams don't renew your CCNA at all, unless you finish all of them before your CCNA expires, and this option is only available to cisco network academy students, and only until the end of July 2004).
If you pass any 642-8xx CCNP examination, your CCNA automatically renews, but if your CCNA expires before you take (and pass) any 642-8xx CCNP examination, you'll need to retake the CCNA exam.
A good reason not to let certs which have to be renewed expire, IMO.