In 3.5, the section "Ex-Monks" (PHB 42) and "Ex-Paladins" (PHB 45) state that multiclassing out of Monk or Paladin means you can never raise Monk or Paladin again.
In looking at PF last night, I didn't see this restriction listed in the PF class description. Is it really true? Can Monks and Paladins really moonlight in another class without giving up all chance for advancement at their day jobs?