Matt Blaze has posted a blog entry following a visit to the Titan Missile Museum
that's just south of Tucson, Arizona. It's a well written summary of the place.
Blaze talks a bit about Titan, PALs, and the "butterfly switch;" mechanisms intended to prevent an unauthorized launch. The Titan system didn't have PALs. The butterfly switch, also known as the "Coded Switch System" (CSS), authorizes the launch. PALs were first required on overseas nukes starting in 1962. Titans were never overseas, and the system was already under construction in the continental US by the time the PAL idea arose.