I heard a radio advertisement for this yesterday and just signed up for it - Wireless Amber Alerts. It say that they will not sell your phone number to any third parties - good!
It has you enter in 5 zip codes that it will send you notification if an Amber Alert has been issued for that area. You can always change it or unsubscribe if you want. But I think it's a good thing to know and you could be help ful to other parents who just had their child abducted. That has to be the most awful feeling in the world.