One of the makers of the H1N1 vaccine (Sanofi Pasteur) has announced that some of their vaccine was not as potent as it should have been. The vaccine in question is for children 6 to 35 months old. They have no concerns about the safety of the vaccine, just the potency. Children in this age range should be getting two doses of the vaccine (1 month apart), so with both shots they should be protected.
Bottom line: if your child is between 6-35 months, make sure they get their booster dose.
You can read more about it here.