Child Mattress Emissions - Check out this site for all sorts of information on chemicals and mattresses. You can even find testimonials from people who have had health problems directly related to their mattresses.
People for Clean Beds - Read about the chemicals in mattresses.
HealthyStuff.org - Great place to check the chemical content of your child's toys, car seat, and your own car.
Cosleeping.org will give you a list of the most important links for information and studies on the subject.