I may take heat for saying this, but I really don't agree with it.
I think it violates some people's privacy. If we are making them post little signs on their lawn, why not paint "SEX OFFENDER" on their doors in huge red letters? Keep in mind, a lot of people who are sex offenders aren't pedophiles, sometimes their charges aren't that serious (ex- 20 year old who gets in trouble for being with a 17 year old), and they've served their time. Besides we already have web databases you can look people up in directly.
My other issue with it is that it gives parents a false sense of security when letting their kids trick-or-treat.
Really, the best way to combat children being preyed on is to always have one (or more) trust-worthy adult with a group of kids when they go out. It also prevents other bad things from happening (ex- kids getting into mischief, kids getting bullied by bigger kids, kids getting hit by cars, etc...).