SSN and passport

[nobody@hidden.net (Anonymous)]

     I recently applied for a U.S. passport and, as an experiment,
entered a SSN of 000-00-0000.  The nice lady at the counter informed
me that it would not be processed unless I provided a valid SSN (which
gets forwarded to the IRS).  It was my understanding that this
disclosure was voluntary and that another form was available for
notifying the IRS that a passport had been applied for.

     My travel plans didn't allow me the luxury of delaying the
application process, so I provided the damn number.  Does anyone know
the real rules for this situation?  Must passports be cleared with the
tax goons or do they just want to keep tabs on people travelling
abroad?